Microbiology of milk: a. Common micro-organisms found in milk b. Fermentation and spoilage of milk c. Milk borne diseases

Microbiology of milk: 

a. Common micro-organisms found in milk 

b. Fermentation and spoilage of milk 

c. Milk borne diseases 

a. Common micro-organisms found in milk :

Milk is a highly nutritious food that can be obtained from a variety of animal sources such as cows, goats, sheep and buffalo, as well as humans, for human consumption.

However, the high nutrient content of these milks, which includes proteins, fats, carbohydrates, vitamins, minerals and essential amino acids , all at a near neutral pH and at a high water activity, provides an ideal environment for the growth of many microorganisms.

Some of these nutrients are directly available to all microorganisms, while others are provided following the metabolism of major components by specific populations to release components and metabolites that are used by others .

It is generally accepted that the lactic acid bacteria (LAB), a group of bacteria that ferment lactose to lactate, are a dominant population in bovine, goat, sheep and buffalo milk, prior to pasteurisation.

The most common LAB genera in milk include Lactococcus, Lactobacillus, Leuconostoc, Streptococcus and Enterococcus. 

Psychrotrophic populations, which particularly establish themselves during cold storage, are also a major component and frequently include Pseudomonas and Acinetobacter spp. 

Other strains of non-LAB genera are also encountered in milk, as well as various yeasts and moulds.

 Human milk on the other hand is typically dominated by Streptococcus, Staphylococcus, Lactobacillus and Bifidobacterium spp.

Cows’ milk:

Typically, cows’ milk contains a significant LAB population that includes Lactococcus , Streptococcus , Lactobacillus , Leuconostoc and Enterococcus spp. 

A number of other microorganisms can be present in significant proportions. These include psychrotrophs, such as Pseudomonas, Acinetobacter and Aeromonas spp., which flourish during cold storage

A number of other microorganisms that had previously been associated with raw milk, including Acinetobacter, Aeromonas, Brevibacterium, Corynebacterium, Lactobacillus, Pseudoalteromonas, Pseudomonas and Staphylococcus.

Goats’ milk :

Goats’ milk also differs from cows’ and sheep’s milk by virtue of having greater levels of iron bioavailability  as well as containing smaller fat globules, having a higher content of fatty acids and forming a softer curd during subsequent fermentations, in turn leading to greater digestibility.

Goats’ milk is most frequently used for cheese making, usually at farm level or in small dairies.

Goats’ milk cheeses are particularly common in south-east Europe .

Goats’ milk is also typically dominated by LAB, including species of Lactococcus , Lactobacillus , Leuconostoc and Enterococcus , as well as Enterobacteriaceae, Micrococcaceae, moulds (filamentous fungi) and yeasts.

In addition to microorganisms commonly encountered in milk,  some species were identified that are not typically associated with goats’ milk or that had previously only been associated with cheeses, including a number of corynebacteria and brachybacteria. 

Another unexpected finding was the presence of several halophilic species not previously associated with milk, including Jeotgalicoccus psychrophilus, Salinicoccus sp., Dietzia maris, Exiguobacterium, Ornithinicoccus sp. and Hahella chejuensis.

Milks that collected during winter were dominated by the presence of Lactococcus and Pseudomonas, those from summer by Pantoea agglomerans and Klebsiella and those from autumn by Chryseobacterium indologenes, Acinetobacter baumannii, Staphylococcus, Corynebacteria and yeasts.

Sheep milk :

Sheep milk is rarely consumed but still constitutes 1.3% of global milk production.

Sheep milk is dominated by LAB, with mesophilic bacteria  &psychrotrophic populations.

Studies assessing the impact of storing sheep milk at refrigeration temperature highlighted increases in psychrophiles, but also in mesophiles. Unsurprisingly, the thermoduric population did not increase .

Other bacteria that have been detected on occasion can include microorganisms of concern from a milk safety perspective including E. coli, Salmonella, Staphylococcus aureus, Bacillus and Clostridium perfringens.

Buffalo milk :

Buffalo milk is consumed in various countries around the world, with India and Pakistan being the highest consumers .

The microbial content of raw buffalo milk has been assessed, through culturing, and found to contain a large population of LAB, including lactococci and lactobacilli, as well as coliforms, E. coli, S. aureus and bacterial endospores.

The dominant microorganisms in the milk were Lactococcus spp., Acinetobacter spp. , Pseudomonas spp. , Streptococcus macedonicus  and Lactococcus lactis . 

A number of other microorganisms were detected in low abundance including Brochothrix, Carnobacterium, Chryseobacterium, Clostridium, Corynebacterium, Enterobacteriaceae, Gammaproteobacteria and Haloanella.

Milk from other animal sources :

Other milks that are consumed by humans around the world include those produced by camels, yaks, donkeys .

Camel milk :

is commonly consumed in African and Arab countries.

The microbial population of camel milk, like that of other milks, can play a role in subsequent fermentations, health promotion and milk spoilage. The milk is typically dominated by mesophiles, including lactobacilli (such as Lactobacillus helveticus, L. casei ssp. casei and L. plantarum), lactococci (such as Lactococcus lactis ssp. lactis), streptococci (such as S. salivarius) and leuconostoc.

yak milk:

Another animal that is typically associated with difficult climates is the yak.

Agin the LAB dominate, with yeasts and coliforms also being common . 

Other studies have identified L. casei, Lactobacillus delbrueckii ssp. bulgaricus, Lactobacillus fermentum, Lactobacillus kefiranofaciens, L. plantarum ssp. plantarum, L. brevis, Lactobacillus buchneri, Leuconostoc lactis, L. mesenteroides, Lactococcus lactis ssp.

 

b. Fermentation and spoilage of milk :

The specific composition of the milk microbiota directly impacts on the subsequent development of dairy products . 

Microorganisms can bring about the fermentation of milk through the production of lactate and have a variety of different impacts on the sensory, texture, flavour and organoleptic properties of resultant products .

Microorganisms can also negatively impact on milk quality and shelf life; for example, psychrotolerant bacteria can proliferate during refrigeration and, through the production of extracellular lipases and proteases, result in spoilage .

The microbial composition of milk can also have healthrelated implications in that the consumption of raw milk contaminated with pathogens can lead to, in some cases, severe illness.

In contrast, it is claimed that other raw milk microorganisms can contribute to health by aiding digestion or by reducing the frequency of allergies, including asthma and atopic diseases, in individuals who consume raw milk during the early years of life.

Milk spoilage is an indefinite term and difficult to measure with accuracy. This uncertainty can cause suffering for both milk manufacturers and consumers. Consumers who have been misled by ambiguous expiration dates on milk cartons waste resources by disposing of unspoiled milk or experience discomfort from drinking spoiled milk. Consumers are often unwilling to purchase products close to their inaccurate expiration dates. This consumer behavior has a negative financial impact on milk producers.

Bacteria growth varies from one species of bacteria to another. While one bacteria species may prosper under certain conditions, another species may weaken. These conditions are interdependent and include nutrient availability, moisture, oxygen levels and the level of other gases, the presence of inhibitors, temperature, and pH.

The pH of unspoiled milk is approximately 6.7, a level at which many forms of bacteria thrive . 

At lower pH levels of 4.0 - 5.0, lactic acid bacteria can grow and produce lactic acid.

While these organisms inhibit the growth of many pathogenic bacteria and are also intentionally employed to ferment milk to make other dairy products such as yogurt and cheese, they can also induce undesirable spoilage in certain products.

Coliforms, a common form of bacteria, have been an indicator of the presence of pathogens in assessing the contamination of water as well as dairy products.

Coliforms can cause rapid spoilage in milk because they ferment lactose with the production of acid and gas, and they can also degrade milk proteins.

Escherichia coli is a well-known example of a coliform .

Studies have shown that other properties of milk also promote bacteria growth, such as the high availability of moisture and dissolved oxygen which supports both aerobic and facultative anaerobic microorganisms . 

Temperature is frequently controlled to limit bacteria growth. Extreme heat is lethal to many organisms, such as coliforms, which explains the process of milk pasteurization (63˚C for 30 minutes).

Two types of bacteria exist in pasteurized milk: thermoduric bacteria, which are capable of surviving the extreme heat during pasteurization, and bacteria that originate from unsanitary conditions postpasteurization. 

Psychrotrophs comprise the largest percentage of bacteria in milk and cause spoilage in refrigerator temperatures at or below 7˚C.

Acidity increases as milk spoils; thus, acidity can be quantified to measure milk quality. 

Acidity in dairy products can be expressed in two ways: 1) titratable acidity, which shows total acidity but not acid strength; and 2) hydrogen ion concentration or pH, which indicates acid strength. 

The natural acidity of milk is 0.16% - 0.18%, and samples with higher figures indicate developed acidity . 

At normal levels of pH, the main protein in milk, casein, remains evenly dispersed. At lower levels of pH below 4.6, the protein can no longer remain in solution, so it coagulates due to acid generated from fermentation.

c. Milk borne diseases :

In developed countries such as the United States of America, foodborne illnesses account for 48 million infections per year. 

Developing countries such as India face greater simultaneous challenges particularly since incorrect processing or storage of dairy products can represent a transmission hazard for a large number of pathogens and can be responsible for outbreaks of brucellosis, listeriosis, tuberculosis, etc.

 It is important to recognize the types of germs which can be transmitted through insufficient thermal preparation of milk or milk products or through post-pasteurization contamination, in order to successfully avoid transmission of milk-borne infections.

Bacterial infections:

 The list of bacteria which can be responsible for milk-borne diseases is long and it includes Brucella spp, Campylobacter jejuni, Bacillus cereus, Shiga toxin-producing E. coli (E. coli O157:H7), Coxiella burnetii, Listeria monocytogenes, Mycobacterium tuberculosis, Mycobacterium bovis, Mycobacterium avium subspecies paratuberculosis, Salmonella spp, Yersinia enterocolitica, and certain strains of Staphylococcus aureus which are capable of producing highly heat-stable toxins.

Brucellosis is one classical example of milkborne infection, Brucella spp being transmitted from goats to humans either through direct contact or through the milk of the infected animal, particularly since the appearance and taste of the milk are rarely affected by the presence of the bacteria. Once transmitted to humans, Brucella is responsible for a type of granulomatous hepatitis or an acute febrile illness which can, at times, persist and progress to a chronically incapacitating disease with serious complications.

Coliform contamination ranks high among the most common types of contamination in the dairy industry. Microorganisms such as Escherichia coli, Pseudomonas aeruginosa, Citrobacter spp, Klebsiella spp and Proteus mirabilis can multiply in the normal summer temperatures and hence unpasteurized milk has every chance of containing E coli. Therefore, even nowadays, basic microbiology tests performed on milk or any dairy product are aimed at detecting coliforms.

The mechanism behind staphylococcal enterotoxin gastroenteritis is the production of a heat-stable enterotoxin by certain strains of Staphylococcus aureus.  Humans and dairy cows are the main carriers of this microbe, presenting mucosal or cutaneous lesions such as impetigo or cattle mastitis. Therefore, either the udder of cattle or the hands of milkers can be responsible for passing on the bacteria to milk, and staphylococcal mastitis is known to be prevalent in India even nowadays,with an older study showing that staphylococci were isolated from 61.97% of the bacteriologically-positive samples, appearing to be the main etiological agents of bovine mastitis in India The enterotoxin is very resistant to heating and pasteurization, boiling of the milk for one hour leading to a decrease in the quantity of toxin but only autoclaving at 15 psi for 20 minutes being able to completely destroy the toxin. The sterilized milk needs to be refrigerated at 0°C to 4°C until further processing. Since staphylococci are known to grow well on saline media, the risk for contamination is higher with home-made salted cheeses.

Typhoid and paratyphoid fever are generally recognized as food-borne and water-borne illnesses but milk-borne infections have also been reported. The source of infection is generally a human carrier among dairy industry workers. Pasteurization is the best way of destroying Salmonella typhi and paratyphi.

Botulism caused by Clostridium botulinum and cholera caused by Vibrio cholerae are rarely transmitted through milk.

Another relatively rare milk-borne pathogen is Bacillus anthracis, a Gram-positive, sporeforming rod which has been shown to pass into the milk when it is present in cattle in large amounts.

Another well-known disease, less frequent since the advent of vaccination, is diphtheria. Corynebacterium diphtheriae can contaminate milk during the handling process if infected dairy workers sneeze or cough the bacilli into milk. Fortunately, the bacteria can be destroyed through high-temperature-short-time (HTST) pasteurization but milk can also be contaminated post-pasteurization.

Viral infections :

A series of viruses can also be involved in milk-borne infections, particularly in developing countries with low sanitary conditions. 

Certain viruses may require heat inactivation temperatures slightly higher than those maintained during pasteurization (for example LTLT applies 61.5°C for 30 minutes while HTST applies 71°C to 72°C for 15 seconds) but generally speaking, the contamination appears to take place post-pasteurization in most developed countries.

Hepatitis viruses, particularly hepatitis A virus (HAV) and hepatitis E virus (HEV) can also contaminate milk and a relatively recent study has demonstrated that increased fat content of dairy products appears to contribute to the heat stability of HAV.

Fungal infections :

A series of pathogenic fungi can infect the udder of the cow and hence be excreted in large amounts in the milk. 

Nocardia asteroides has been found to cause bovine mastitis,being excreted in milk for a period of several months. 

This fungus survives even if the milk is treated at a temperature of 74°C for 15 seconds or at 64°C for 30 minutes, but complete destruction of the organism is possible when the milk is heated at 66°C for 30 minutes. 

Other fungal species such as Nocardia brasiliensis, Candida tropicalis, Candida albicans or Candida krusei have also been shown to cause bovine mastitis and therefore can be transmitted to humans through incorrectly processed milk, posing a threat of fungal infection particularly in immunodepressed patients (for example in case of diabetes, HIV-positive patients with decreased CD4 count,patients with cirrhosis or with chronic alcohol consumption).

Parasitic infections : 

Certain parasites such as Taenia spp or Toxoplasma gondii can contaminate milk and be transmitted to humans. 

Other sources of infection include the environment of milk procurement, which is heavily controlled in industrialized farms. Soil contamination may also lead to the presence of soil-borne parasites in milk (e.g., Ascaris lumbricoides, Trichuris trichiura).