Chapter 73
Bacteriology of Water, Milk, Air and Food
Introduction
Drinking water or potable water, ideally, should not only be safe but also pleasant to drink. It should be clear, colourless and devoid of disagreeable taste or smell. It should be free from pathogenic microorganisms and chemical substances.
On the other hand milk remains sterile at secretion from the udder but immediately gets contaminated even before it is secreted out of the udder. Apart from mastitis cases, these milk-contaminant bacteria are found be harmless and occur in very less numbers. In addition, infection of the milk by microorganisms may occur during milking, handling, storage and all the downstream pre-processing events.
Innumerable microorganisms are present all over in air and in the environment. Apart from bacteria, moulds and viruses are also present and can be transmitted from person to person in the form of aerosols. The proportion of the dust particles or aerosols reaching the lung depends on their size. All particles over 5 μm are retained in the nose and those of 1 μm reach the lung and are retained in the alveoli.
Bacterial analysis of water, milk and air is therefore carried out to indicate quality of these items used in day-to-day life.
Bacteriology of Water
Water is said to be contaminated or polluted when it is contaminated with sewage or other excreted matter from humans and animals that contains infective and parasitic agents, poisonous chemical substances and industrial or other wastes.
Bacterial Flora in Water
Bacterial flora in water (Table 73-1) can be classified into three groups as follows:
Natural water bacteria: These are the bacteria that are commonly found in water free from gross pollution.
Table 73-1
Bacterial Flora in Water
| Source of Water | Bacteria |
| Natural water bacteria | Micrococcus, Pseudomonas, Acinetobacter, Serratia, Alcaligenes and Flavobacterium |
| Soil bacteria washed into water | Bacillus subtilis, Enterobacter cloacae and Enterobacter aerogenes |
| Sewage bacteria | |
| Proper sewage bacteria | Clostridium perfringens, Proteus species |
| Intestinal flora through sewage | Escherichia coli, Enterococcus faecalis, Proteus species, Klebsiella species, Clostridium perfringens |
Soil bacteria: These are the bacteria that are not normal inhabitants of water but are found after being washed into the water during heavy rains.
Sewage bacteria: These are the bacteria that are not normal inhabitants of water but are found in water after being contaminated with sewage. These bacteria include those which are the normal inhabitants of the intestine of humans and animals. These also include the bacteria that live mainly on decomposed organic matter of either plant or animal origin.
Factors Determining the Number of Bacteria in Water
The following factors determine the number of bacteria in water:
Salinity: The number of bacteria present in water depends on salinity of water; more is the salinity, lesser is the number of bacteria. However, some halophilic bacteria survive better in saline water.
Acidity: Acidity of water has a deleterious effect on most of the bacteria.
Temperature: Low temperature usually favours survival of the bacteria. However, in the presence of organic materials, bacteria tend to multiply even at high temperature.
Light: Sunlight with the wavelength of 300–400 nm is highly bactericidal, provided water is clear and static. The bactericidal effect is reduced due to the presence of organic matter and due to movement in water.
Storage: Storage of water decreases bacterial count in stored water due to sedimentation and revitalisation.
Organic matter: When organic matter is plenty, the microorganisms tend to multiply and are present in large numbers whereas when it is less, the organisms are few.
Type of water: Surface water is more likely to be contaminated than the deep water. The latter is usually pure.
Water-Borne Microorganisms
Supply of drinking water is contaminated with sewage or other excreted matter from humans and animals; it can transmit a wide number of pathogens that includes bacteria, viruses and parasites (Table 73-2).
Table 73-2
Water-Borne Pathogens
| Bacteria | Vibrio cholerae; Salmonella typhi, Salmonella paratyphi A, B and C; Shigella spp.; Escherichia coli; Yersinia enterocolitica and Campylobacter jejuni |
| Viruses | Hepatitis A virus, Hepatitis E virus, poliomyelitis virus, rotavirus and Norwalk virus |
| Protozoa | Entamoeba histolytica, Giardia lamblia, Cryptosporidium spp., Cyclospora spp., Isospora spp. and Balantidium coli |
| Helminthes | Ascaris lumbricoides, Trichuris trichiura and Enterobius vermicularis |
Bacteriological Examination of Water
A wide number of pathogens can be transmitted by contaminated water. Hence, the main purpose of water testing is to know whether the water is contaminated by water-borne pathogen or not. Therefore, in the interests of public health, water supplies should be tested regularly to confirm their freedom from such contamination.
It is impracticable to make an attempt to detect all types of water-borne pathogens, any of which may be present only intermittently. Instead attempt is made on testing the water for microorganisms that indicate faecal contamination of water. These are known as indicator organisms. These are usually common intestinal commensal bacteria, which are universally present in and excreted in large numbers in human and animal faeces. They are rarely found in other sources.
Indicator Organisms
The presence of indicator organisms specifies that:
• faecal matter has contaminated the water supply,
• the faecal bacteria has not been removed by purification processes and
• hence, the water supply may be a source of contamination with dangerous intestinal pathogens.
The indicator organism should have the following properties:
• It should be present in human faeces in large numbers.
• It should not multiply in water to any extent.
• It should be more resistant than pathogens to the stresses of the aquatic environment and disinfection processes, such as the process of chlorination.
Some of the organisms that are used as indicator organisms are as follows:
Coliforms: These are the bacteria that occur in large numbers in sewage and faeces. They are also found in the environment in the absence of faecal contamination. Traditionally, these are members of the Enterobacteriaceae, which grow in the presence of bile salts and produce acid and gas from lactose within 48 hours at 37°C. The total coliform count is widely regarded as the most reliable indicator of potable water quality.
Faecal streptococci: These bacteria are found in the intestinal tract of humans and animals. They belong to the genus Enterococcus, and are catalase-negative and Gram-positive cocci. They can survive at 60°C for 30 minutes, and can grow at a pH of 9.6 and in the presence of 6.5% sodium chloride.
Sulphite-reducing clostridia: These are members of the genus Clostridium, which can reduce sulphite to sulphide. The most important of the group is C. perfringens. Although it is less numerous in human faeces than other indicator organisms, its spores can survive in the environment and treatment processes better than most of other indicator bacteria.
Collection of Water Samples for Analysis
Water samples should be collected in heat-sterilised bottles of 250 mL capacity with ground glass stoppers. A 0.23 mL of fresh 1.8% solution of sodium thiosulphate crystal is added into the bottle before sterilisation, to neutralise the bactericidal effect of any chlorine or chloramine in the water.
Sampling from a tap or pump outlet: When collecting from taps, allow the water to run to waste for 2–3 minutes before running it into the bottle. Then the stopper is opened, the bottle is filled with water and the stopper is replaced.
Sampling from a reservoir, such as stream, river, lakes and tanks: When sampling from streams or lakes, the bottle is opened at a depth of about 30 cm with its mouth facing the current and it is ensured that the water entering the bottle has not been in contact with hands.
Sampling from a well: The sampling bottle is tied with a stone and a clean cord of suitable length and is lowered to the required depth in the well. The bottle is completely immersed in the water. When the bottle is completely filled, it is pulled out and then it is stoppered. It is ensured that the bottle does not touch the side of the well at any time. The bottle, after collection of water, is wrapped by a Kraft paper and is labelled with following details: the source, time and date of collection of water samples. The bottle is transported immediately or preferably within 6 hours of collection in a cool container being protected from the sunlight.
Methods of Water Analysis
Following tests are generally done for the routine bacteriological analysis of water:
1. presumptive coliform count,
2. differential coliform count,
3. membrane filtration method,
4. detection of faecal streptococci and C. perfringens and
5. detection of specific pathogens.
Presumptive coliform count: The test is called presumptive coliform count because the reaction demonstrated may occasionally be due to the presence of some other microorganisms. This is a multiple tube method in which measured volumes of water and dilutions of water are added to a series of tubes or bottles containing a liquid indicator medium. The most common medium used is double-strength and single-strength MacConkey broth containing bromocresol purple and an inverted Durham tube to detect the production of gas (Fig. 73-1).
Fig. 73-1 Multiple tube method.
The measured volume of water to be tested is added by sterile graduated pipettes to tubes or McCartney bottles as follows: (1) 50 mL of water is added to one bottle of 50 mL double-strength medium, (2) 10 mL of water each to five bottles of 10 mL double-strength medium, (3) 1 mL of water each to five tubes of 5 mL single-strength medium and (4) 0.1 mL of water each to five tubes of 5 mL single-strength medium.
The inoculated bottles are incubated at 37°C for 48 hours. The presumptive coliform count per 100 mL of water is determined by demonstration of acid and gas in MacConkey broth using the McCrady probability in Table 73-3. The water sample showing a presumptive coliform count of 0 is considered as excellent, whereas water samples showing coliform counts of 1–3, 4–10 and more than 10 per millilitre are considered as satisfactory, suspicious and unsatisfactory, respectively.
Table 73-3
McCrady Probability Table
| Quantity of Water | 1 tube of 50 mL | 5 tubes of 10 mL | 5 tubes of 1 mL | Probable No. of Coliform Bacilli in 100 mL Water |
| No. of samples of each quantity tested | 1 | 5 | 5 | |
| No. of samples giving positive reaction (acid and gas) | 0 | 0 | 0 | <1 |
| 0 | 0 | 1 | 1 | |
| 0 | 0 | 2 | 2 | |
| 0 | 1 | 0 | 1 | |
| 0 | 1 | 1 | 2 | |
| 0 | 1 | 2 | 3 | |
| 0 | 2 | 0 | 2 | |
| 0 | 2 | 1 | 3 | |
| 0 | 2 | 2 | 4 | |
| 0 | 3 | 0 | 3 | |
| 0 | 4 | 0 | 5 | |
| 1 | 0 | 0 | 1 | |
| 1 | 0 | 1 | 3 | |
| 1 | 0 | 2 | 4 | |
| 1 | 0 | 3 | 6 | |
| 1 | 1 | 0 | 3 | |
| 1 | 1 | 1 | 5 | |
| 1 | 1 | 2 | 7 | |
| 1 | 1 | 3 | 9 | |
| 1 | 2 | 0 | 5 | |
| 1 | 2 | 1 | 7 | |
| 1 | 2 | 2 | 10 | |
| 1 | 2 | 3 | 12 | |
| 1 | 3 | 0 | 8 | |
| 1 | 3 | 1 | 11 |
Differential coliform count: Differential coliform count is carried out by Eijkman test. Some spore-bearing bacteria give false-positive reactions in the presumptive coliform test. The Eijkman test is used to determine whether the coliform bacilli detected in the presumptive test are E. coli or not. The test is carried out after the usual presumptive test and is performed by inoculating sub-cultures from the positive presumptive tests at 44 and 37°C into fresh tubes of single-strength MacConkey broth with Durham tube. Those tubes showing gas in Durham tube after incubation at 44°C after 24 hours indicate the presence of E. coli. Confirmation of E. coli is made by sub-culturing on the solid media and testing the colour for indole production and citrate utilisation (Fig. 73-2).
Fig. 73-2 Eijkman test.
Detection of faecal streptococci: Detection of faecal streptococci in water sample also indicates faecal contamination of water.
The test is carried out after the usual presumptive test and is performed by incubating sub-cultures from the positive presumptive tests at 45°C for 18 hours. The presence of acid in the medium indicates the presence of E. faecalis. Further confirmation of E. faecalis is made by sub-culturing on the MacConkey medium.
Detection of C. perfringens: It is detected in water samples by inoculating it in litmus milk medium and incubating it anaerobically at 37°C for 5 days. A typical stormy clot reaction along with formation of acid indicates the presence of C. perfringens. Further confirmation of the bacteria is made by motility and nitrate reduction test.
Membrane filtration method: In this method, a measured volume of the water sample is filtered through a membrane with a pore size small enough to retain the indicator bacteria to be counted. The membrane is then placed and incubated on a selective indicator medium, so that the indicator bacteria grow to produce colonies. These colonies are then identified and counted to give a measure of the contamination of the water.
Detection of specific pathogens: V. cholerae, S. typhi, etc., are the specific pathogens, which can be detected in water sample to know the contamination. Specific selective media for these bacteria are employed to detect these in contaminated water. For example, double-strength selenite F broth is used for S. typhi, whereas alkaline peptone water is used for V. cholerae. Subsequently, these bacteria are identified by their cultural and biochemical characteristics.
Viruses of Water
Over 100 enteric viruses and other cytopathogenic viruses (e.g. hepatitis A virus, hepatitis E virus, poliomyelitis virus, rotavirus and Norwalk virus) are said to be present in both sewage and drinking water. Viruses can be removed from water by either coagulation or disinfection procedures. By chlorination, with a concentration of 0.5 mg/L free residual chlorine, pH < 8, turbidity of 1 or less nephelometric turbidity units and minimum contact period of 30 minutes, viruses are assumed to be destroyed. Although chlorination is used as the best possible treatment for removal of viruses, the exact mechanism is unknown.
Protozoa in Water
Protozoans are also said to be the most common contaminants of drinking water. There are no sensitive indicators for protozoal contamination of water. And coliform counts are also not reliable indicators as these protozoans are found to be resistant to normal levels of chlorine used for disinfection.
Bacteriology of Milk
Milk is an opaque white liquid, which provides the primary source of nutrition for newborns before they are able to digest other types of food. It’s a water-based fluid which is primarily an emulsion of butterfat globules. Every single fat globule encompasses a membrane made up of phospholipids and proteins. These emulsifiers prevent the individual globules from attaching with each other into observable grains of butterfat and also protect the globules from the fat-digesting activity of enzymes found in the fluid portion of the milk.
Bacterial Flora of Milk
Lactic Acid Bacteria
This group of bacteria has the capacity to produce lactic acid from lactose. They are generally present in the milk and they have been used as starter cultures in manufacturing of cultured dairy products like yoghurt. Some of the lactobacilli present in milk are mentioned as follows:
1. Lactococci, such as Lactic delbrueckii subsp. lactis (Streptococcus lactis) and Lactococcus lactis subsp. cremoris (Streptococcus cremoris),
2. Lactobacilli, such as Lactobacillus casei, Lactobacillus delbrueckii subsp. lactis (L. lactis), Lactobacillus delbrueckii subsp. bulgaricus (Lactobacillus bulgaricus) and Leuconostoc.
Coliforms: These are facultative anaerobes which grow at an optimum temperature of 37°C. They are indicator organisms which are closely correlated with the existence of pathogens but are not compulsorily pathogenic themselves. They are capable of spoiling milk rapidly as they are able to ferment lactose by producing acid and gas, and are able to disintegrate proteins in milk. They can be killed by pasteurisation; hence, their presence after pasteurisation is usually clear indication of contamination. E. coli is an example which can act as a milk-contaminating microbe.
Importance of Presence of Microorganisms in Milk
The data about the microbial content in milk can help to assess its sanitary quality and the sterile conditions maintained during its production. Bacteria present in the milk can cause spoilage of the product if allowed to multiply. Milk is easily susceptible to be contaminated with pathogenic microorganisms. Precautions must be taken, therefore, to prevent this possibility and to eradicate pathogens that may gain access. Certain microorganisms produce changes chemically that are beneficial in the production of dairy products, such as yoghurt and cheese.
Microorganisms Causing Spoilage of Milk
The quality of microbes in raw milk is vital for the production of dairy foods at the utmost quality. A term which describes the degradation of a food’s texture, odour, flavour or colour, to the extent where it is unappetising or unacceptable for human consumption is called as spoilage. The spoilage of food by microbes often involves the disintegration of protein, carbohydrates and fats by the either microorganisms itself or by their enzymes.
Psychrotrophic organisms are the microbes that are basically involved in spoilage of milk. Most of the psychrotrophic organisms are killed by pasteurisation temperatures; anyhow, some microorganisms like Pseudomonas fluorescens, Pseudomonas fragican produce lipolytic and proteolytic extracellular enzymes, which are stable in heat and capable of producing spoilage. Some species such as Arthrobacter, Bacillus, Clostridium, Corynebacterium, Microbacterium, Micrococcus, Lactobacillus and Streptococcus species have the capacity to withstand pasteurisation and can grow at refrigeration temperatures, thus can cause spoilage problems.
Pathogenic Microorganisms in Milk
Hygienic production of milk involves the proper handling and storage of milk, and compulsory pasteurisation shall decrease the hazard of milk-borne diseases, such as brucellosis, tuberculosis and typhoid fever. There have been reports of a number of food-borne illnesses which resulted due to the various issues such as ingestion of raw milk or poorly handled dairy products or not properly pasteurised, which cause post-processing contamination of milk. Milk-borne diseases are of three types:
• infections primarily of humans but transmitted through milk,
• infections primarily of animals that can be transmitted to humans and
• infections transmitted by milk contaminated with excreta of ticks and rats (Table 73-4).
Table 73-4
Milk-Borne Diseases
| Source | Diseases |
| Primarily of humans | Enteric fever, cholera, bovine tuberculosis, shigellosis and staphylococcal food poisoning |
| Primarily of animals | Tuberculosis, brucellosis, salmonellosis, cowpox, Campylobacter fetus infection and Yersinia enterocolitica infection |
| Contamination of milk by ticks and rats | Tick-borne encephalitis and Streptobacillus moniliformis infection |
It should be remembered that moulds of the species such as Aspergillus, Fusarium and Penicillium can also grow in milk and dairy products. If the conditions are permissible, these moulds produce mycotoxins which cause a serious health hazard.
Bacteriological Examination of Milk
Bacteriological examination of milk can be carried out by following groups of tests:
1. colony counts,
2. coliform counts,
3. chemical tests, such as methylene blue reduction test, phosphatase test and turbidity test and
4. detection of specific pathogens.
Colony Counts
This test is carried out by plate dilution methods. Raw milk may contain 500 to several million bacteria per millilitre of milk.
Coliform Counts
The presence of coliforms in milk indicates improper pasteurisation of milk, or post-pasteurisation contamination of milk. This is because all coliforms are destroyed during the process of pasteurisation of milk. This test is carried out by inoculating varying dilutions of milk into MacConkey medium and noting the production of acid and gas after 48 hours of incubation at 37°C. The presence of acid and gas indicates the presence of coliforms in milk.
Chemical Tests
Methylene blue test: This is a test used since long to demonstrate bacterial contamination of milk. It is an indicator of the number of viable bacteria present in the milk. It is a rapid and inexpensive way of indicating poor-quality milk that had been unrefrigerated. The basis of the test is that the presence of viable bacteria in milk reduces methylene blue and decolourises the milk when kept in a dark place. The test is performed by using a 1:300,000 solution of methylene blue. One millilitre of the methylene blue solution is added to 10 mL of the milk sample in a test tube. Both the milk and methylene blue solution are mixed by shaking and then placing the mixture in a water bath at 37°C for 30 minutes in dark. Untreated milk can be considered as satisfactory if it fails to decolourise the dye within 30 minutes.
Phosphatase test: Alkaline phosphatase is normally present in milk and is inactivated if pasteurisation has been carried out effectively. Successful pasteurisation, which kills non-sporing pathogens, also inactivates alkaline phosphatase. The test depends on the ability of the enzyme to liberate p-nitrophenyl phosphate after breaking down disodium p-nitrophenyl phosphate. The test determines the amount of alkaline phosphatase present after pasteurisation by measuring the amount of p-nitrophenyl phosphate it liberates, which is known by development of a yellow colour that is quantitated by a colorimeter.
Turbidity test: This is the definitive test for checking the sterilisation of milk, thereby distinguishing it from the untreated milk and milk that has been merely pasteurised. The degree of heating necessary for sterilisation causes all the heat-coagulable proteins in milk to become precipitable by ammonium sulphate. If the amount of heat applied to milk is insufficient for sterilisation, some of its protein will not be precipitated by ammonium sulphate and will be detected by its coagulation, resulting in turbidity when a filtrate of ammonium sulphate treated-milk is boiled. The absence of turbidity indicates that the milk has been boiled or heated to at least 100°C for at least 5 min.
Detection of Specific Pathogens
Mycobacterium tuberculosis and Brucella species are the specific pathogens that can be detected in milk to know transmission of these bacteria through milk. Specific selective media for these bacteria are employed to detect these in infected milk. For example, M. tuberculosis can be isolated from centrifuged deposit of milk by inoculation in Lowenstein–Jensen (LJ) medium or by inoculation in guinea pigs. Similarly, Brucella spp. may be isolated from milk samples by inoculating in serum dextrose agar or by intramuscular inoculation in guinea pigs. Subsequently, these bacteria are identified by their cultural and biochemical properties. Brucella spp. in infected animals can also be demonstrated by milk ring test, whey agglutination test and demonstration of Brucella antibodies in serum.
Bacteriology of Air
The load of microorganisms present in the air depends on whether the air is indoors or outdoors. The number of bacteria at any time is dependent on many factors, the most important of which are the number of persons present, the amount of their body movements and the amount of disturbance of their clothing.
Bacteriological Examination of Air
Air-borne infection denotes the infection caused by transmission of respiratory droplets or aerosols less than 5 μm size. Droplet infection denotes the infection caused by transmission of respiratory droplets or aerosols larger than 5 μm size.
Observations of the number of bacteria carrying particles in the air may be required in premises where safe working depends on the air’s content of bacteria being kept at a very low level, for example, operation theatres. Bacteriological examination of air is also necessary for monitoring air quality in hospital wards, storehouse of food and pharmacy, etc. Ideally, bacterial count should not exceed 1 per cubic foot of air in operation theatre for neurosurgery, 10 per cubic foot in operation theatre for other surgery and 50 per cubic foot in homes, offices and factories.
Microbial Quality of Outdoor Air
The microbial quality of outdoor air depends on various factors such as: (1) the density/mass of both human and animal populations, (2) the nature of vegetation/plantations, (3) the content of the soil and (4) atmospheric conditions including humidity, temperature, wind, rainfall and sunlight.
The four main areas that are known to contribute to the presence of pathogens in the outdoor air. The occurrence of these microbes have been associated with human health are natural environments, engineering environments, agricultural and waste treatment plants. Fragments of moulds and spores naturally occur high in number than bacteria in atmosphere. Most of the bacteria that may be present in atmosphere are non-pathogenic and even if some pathogen are present, they may just contaminate the air and are incapable of causing any disease due to the adverse conditions of the outdoor air. It is also known that pathogenic microbe does not multiply in air. Aerobic spore-bearing bacilli and some Achromobacter, Sarcina and Micrococcus found in the upper atmosphere are mostly isolated from soil and surface dust and it can be carried over for miles. Some infective microbes are carried only for short distances ad during the course their infectious ability is diminished, except in rare cases such as the foot and mouth disease virus.
Microbial content of indoor air: This may contain particles of biological origin suspended in air in an indoor environment. This depends on the following sources:
1. Dust particles of varying sizes originating from animals (pets), plants or vegetables, ventilation or air conditioning system.
2. Humans are the important source of common pathogens. Nasal secretions dispersed by fingers/hand to any other part of the body or to the skin, bedding or clothing forms dust particles when detached. Pathogenic microbes form dust particle when detached directly from different parts of the body or from the skin, for example, from septic wounds, perineum, etc. Intestinal pathogens can be disseminated through dried faecal particles from napkins used for infants. Heavy particles fall to ground and forms settled house dust, while smaller particles of 1 mm or less in diameter remains suspended in air. The suspended biological particles are commonly associated with respiratory illness in human. The layer of air surrounding the human body serves as a source of pathogenic microbe. Physical activity liberates the desquamated epithelial cells into the environment which also contributes to the microbes in dust. Hospital wards, where patients are treated, are also sources of potential infectious agents like haemolytic streptococci, staphylococci, tubercle bacilli, diphtheria bacilli, etc. Bed clothes are also a rich source of pathogen loaded dust. Tuberculosis, different forms of bacterial pneumonia, coccidioidomycosis, influenza, measles and gastrointestinal illness are known to be caused by indoor aerosols.
3. Droplets and droplet nuclei: Human generates varying number of droplets, during the activities of talking, sneezing and coughing. These droplets are expelled from the body that ranges in size from less than 1–15 mm (Table 73-5). Based on the different sizes, these particles may fall to the ground or stay suspended in air. In the process, it might get evaporated; smaller the particle, faster is the evaporation rate and vice versa. Droplet nuclei are formed when droplets are converted to minute particles during evaporation. The viability of the microbes in the droplet nuclei depends on numerous factors. Particles of 5 mm size are retained in nose while those of 1 mm reach the lung and are retained in the alveoli. Droplets containing infectious agents may also be released from various laboratory practices, dental manipulations, washing or flushing of water closets.
Table 73-5
Pathogenic Air-Borne Microorganisms
| Property | Transmission by Air-borne | Transmission by Droplet |
| Droplet size | Less than 5 μm | More than 5 μm |
| Droplet origin | Produced during invasive procedures and also during coughing, sneezing, talking | Produced during invasive procedures like bronchoscopy, suction aspiration and sneezing, talking, coughing |
| Features of droplets |
• Transmission occurs through close contact
• Present in air for brief time and travel just for short distance
|
• Immunocompromised individuals can be infected even if less distance from infected person
|
Microorganisms responsible 1. Bacteria
|
• Bordetella pertussis (whooping cough)
• Corynebacterium diphtheria (diphtheria)
• Yersinia pestis (pneumonic plague)
• Haemophilus influenza type B
• Mycoplasma pneumoniae, Streptococcus species (pneumonia)
• Neisseria species (meningitis)
|
Mycobacterium tuberculosis |
|
2. Viruses
|
• Adenovirus
• Rubella virus
• Influenza virus
• Parvovirus B19
• Mumps
|
Measles Varicella-zoster virus Influenza virus |
Various methods have been devised for the measurement of bacterial content of air. A primary distinction must be drawn between the methods that measure the rate at which bacteria carrying particles are settling by gravity on to exposed surfaces and those that count the number of bacteria carrying particles in a given volume of the air. Two types of methods used for bacteriological examination of air are as follows:
1. settle plate method and
2. slit sampler method.
Settle Plate Method
Settle plate method is used for testing bacteriological quality of air in surgical operation theatres and hospital wards. In this method, Petri dishes containing nutrient agar and blood agar (for detecting pathogenic staphylococci and streptococci) are left open for half an hour to 1 hour. During this period of exposure, large bacteria carrying dust particles settle on to the media. The plates are then incubated at 37°C for 24 hours, following which the colonies formed on the media are counted.
Slit Sampler Method
Slit sampler method is most efficient and convenient method used for estimation of the number of bacteria present in a measured volume of air. In this method, 1 ft.3 volume of air is directed onto a plate containing culture medium through a slit 0.25-mm wide. The plate is then rotated so as to allow the microorganisms present in the air to spread out evenly on the medium. The culture medium is incubated and the number of colonies formed on the medium indicates the number of bacteria present in the air.
Bacteriology of Food
One of the major causes of morbidity particularly due to the public health problems results from food-borne infections. Morbidity is not the only problem but sometimes it might result in mortality as well. Several species of microorganisms can contaminate food and it becomes a good medium for growth of these microorganisms.
There are different and variety of aetiological patterns concerning food-borne infections and these patterns may vary around the world. Factors that influence these aetiological patterns include food preferences, ability of the laboratories, public and medical practitioner’s awareness. For example, in the United States more than 50% of the food-borne diseases are caused by Staphylococcus aureus and Salmonella but in the United Kingdom more than 90% of food-borne diseases are caused by C. perfringens. In Japan more than 50% of the diseases are caused by Vibrio parahaemolyticus.
Food-borne outbreak infections refers to a condition when two or more people get the same illness from the same contaminated food or drink. Usually, the illness is restricted to gastrointestinal manifestations and food is incriminated as the source of illness. Food-borne illness, most commonly referred to as food poisoning, occurs due to the consumption of contaminated, spoiled or toxic food. The commonest symptoms of food poisoning are nausea, vomiting and diarrhoea. The causative agents are usually gastrointestinal microorganisms which may also produce harmful toxins.
Source of Food Contamination
Food ingested must be free of microorganisms. When microbes contaminate the food, it indicates lack of sanitation and precautions while handling and preparing cooked food. The food can be contaminated through various ways as follows:
Faeces: The contaminations of faeces in food are mainly due to poor hygiene of food handlers.
Insects: The housefly has been identified as an important vector in the propagation of various infectious diseases such as shigellosis, cholera and salmonellosis. They act as a cross-contamination vector for other food-borne pathogens as well.
Air: The food uncovered may attract a lot of microorganisms in air and also invite dust to settle on it.
Domestic animals: They contaminate through their excreta when handled by people who do not practise proper sanitary precautions.
Laboratory Diagnosis
Laboratory diagnosis of suspected food-borne infection or food poisoning depends on the following:
1. Clinical history: A careful and detailed history is usually obtained to rule out the possible causes altogether.
2. Specimens: Samples include the consumed food as well as vomitus and faeces from affected persons.
3. Detection of specific pathogen: These can be detected by (1) direct microscopic examination of samples and (2) culture of samples. This is also supplemented by detection of bacterial toxins and microbial and chemical analysis of food samples for the possible presence of gastrointestinal pathogens.
Study questions
1. List water-borne diseases. Discuss various methods for bacteriologic examination of water.
2. Describe bacteriological flora of milk. Discuss various methods for bacteriologic examination of milk.
3. Describe various methods for bacteriologic examination of air.
4. Short notes:
a. Bacterial flora of water
b. Factors to determine the number of bacteria in water
c. Settle plate method
d. Methylene blue test
Online study material
Multiple Choice Questions
1. All the following bacteria are indicators of bacterial contamination of water except
A. E. coli
B. Enterococcus species
C. S. aureus
D. C. perfringens
2. The test done for the routine bacteriological analysis of water is
A. Slit sampler methods
B. Turbidity test
C. Phosphatase test
D. Eijkman test
3. All the following bacteria can be normally present in the milk except
A. S. lactis
B. Enterococcus species
C. L. bulgaricus
D. S. cremoris
4. All the following tests are used for detection of bacterial contamination of milk except
A. Eijkman test
B. Methylene blue reduction test
C. Phosphatase test
D. Turbidity test
5. The method used for estimation of number of bacteria in a measured volume of air is
A. Settle plate method
B. Phosphatase test
C. Turbidity test
D. Methylene blue reduction test method
Keys: 1. C; 2. D; 3. B; 4. A; 5. A