Microorganisms present in Raw Milk / Source of Contamination

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Microorganisms in raw milk

There are several types of microorganisms present in raw milk. The number and types of microflora depends on the nature and extent of contamination. Milk procurement under hygienic condition and instant cooling or storage at low temperature below 4 C (degree centigrade denote as C) restrict contamination and proliferation of microorganisms.

Snap of a dairy farm. Pic. courtesy – Unsplash

Types of Microorganism present in raw milk 

Numerous groups of microflora present in raw milk such as mesophilic, thermophilic, thermoduric, psychrotrophic and pathogenic micro organisms. The presence and numbers of individual groups vary with the milk procurement condition and handling. 

Mesophilic (aerobic) micro organisms 

1. Streptococci

Feacal streptococci – it is also called enterococci. e.g. S. faecalis, S. liquefaciens, S. zymogenes. Group N streptococci such as S. lactis, S. cremoris. Mastitis causing streptococci – S. agalactise, S. dysgalactiae, S. uberis (0-50 percent incidence in low count milk sample).

2. Micrococci

Micrococcus spp. such as M. varians, M. caseolyticus, M. flavum, M. freudenreichii, M. roseus etc. Staphylococcus spp. such as S. aureus, S. epidermidis. (30-90 percent incidence in low count milk sample) 

3. Gram positive rod (Asporogenous)

Microbacterium spp. Corynebacterium diphtheriae, C. pyogenes, C. bovis; Mycobacterium spp. such as M. tuberculosis, M. bovisLactobacillus spp. such as L. casei, L. plantarum, L. acidophilus; Arthrobacter and Kurthia spp. (less than 10 percent incidence).

4. Gram positive rods (sporogenous)

Bacillus spp. such as B. cereus, B. subtilis etc. (less than 10 percent incidence) 

Spore forming rod
5. Gram negative rods and coliforms

Pseudomonas spp. such as Ps. fragi, Ps. fluorescens, Ps. aeruginosa, Ps. putrefaciens, Ps. nigrificansAlcaligenes spp. such as A. viscolactis.
ColiformsEscherichia coli, Enterobacter aerogenes, Klebsiella pneumoniae, Serratia marcescens, Flavobacterium maloloris, Flavobacterium arborescens, Acinetobacter viscosymbioticum, Acinetobacter lipopyticum.
(less than 10 percent incidence).

6. Miscellaneous 

Yeasts – Candida, Saccharomyces, Molds – Aspergillus, Mucor, Rhizopus, Penicillium, Geotrichum, Actinomycetes (less than 10 percent imcidence).


1. Mycobacterium tuberculosis is a pathogen microorganism, responsible for tuberculosis or TB disease. 

2. Coliforms are gas producing organism. They might be faecal or non faecal origin. Faecal means – come from human or animal faeces (stool). 

3. Group N streptococci and Lactobacillus spp. are helpful organisms. 

Thermophilic Microorganisms

Thermophilic means heat loving (philic) bacteria. These bacteria can readily grow at 55 C and most of them have an upper limit of growth at about 70 C (Not only survive but also grow at higher temperature). Some thermophilic micro organisms may grow at 37 C which are called facultative thermophilic organism. Entry of thermophilic micro organisms into raw milk – Soil, bedding, feeds amd sometimes from water supplies. 
Common members of thermophiles present in raw milk – Bacillus spp. such as B. stearothermophilus, B. circulans, B. thermoacidurans. Lactobacillus thermophilus. 

Thermoduric Microorganism

Thermoduric micro organisms can survive at higher temperature but can not grow. Thermoduric bacteria can survive at pasteurisation temperature and reduce shelf life of final products made from raw milk. Spoilage or defects of butter, cheese occurs due to thermoduric micro organisms. 

Thermoduric microorganism in raw milk

1. Bacillus spores

Bacillus spore count is generally higher in winter than in summer. It mainly comes from teat surface of milch animal which is contaminated with bedding materials. B. licheniformis, B. cereus spores are mainly counted in raw milk.

2. Microbacterium

It shows 100% survival at pasteurisation temperature (63 C for 30 minutes). Microbacterium lacticum generally comes from milking equipment.

Unclean milk can leads microbial population in raw milk

3. Clostridium spores

Clostridium spores mainly comes from silage and bedding materials. The number of Clostridium spores higher in winter season. The number of Clostridium spores decline when cows go for pasture. e.g. Clostridium tyrobutyricum.

4. Micrococcus spp. 

Micrococcus almost exclusively comes from utensils and milking equipments, cans for milk collection. Almost 100% survival of pasteurisation temperature. e.g. Micrococcus freudenrichii, M. caseolyticus etc. 



5. Alcaligenes

Alcaligenes tolerans is a common example. 1 to 10% strains of this species survive pasteurisation temperature. 

6. Others 

Lactobacilli, Streptococci may survive upto 60 C for 20 minutes. e.g. S. faecalis, S. thermophilus.

Psychrotrophic Microorganisms

Psychrotrphic micro organisms are those which can grow at commercial refrigeration temperature i. e. 2 – 7 C. Although it is not their optimum temperature for growth. Psychrophiles – that have an optimum temperature below 20 C under Psychrophiles, majority of Psychrotrphic bacteria are mesophilic (optimum growth temperature is 20 to 30 C) in nature. 

Psychrotrphic bacteria belongs to two broad catagories such as gram negative rods and gram positive rods/cocci. 

Gram negative rods –

Pseudomonas spp., Acinetobacter, Achromobacter, Flavibacterium, Alcaligenes and Enterobacter (Coliforms). 
Pseudomonas spp. can produce heat stable enzymes such as proteinases and lipases. During pasteurisation vegetative cell may be destroyed but heat stable enzymes could not get deactivated and responsible for fat and protein degradation causing spoilage of milk. Improper cleaning of equipments, utensils are major sources of contamination of these group of micro organisms.

 Gram positive rod or cocci

Bacillus spp. such as B. coagulans, B. circulans and B. subtilis
Arthrobacter spp., Streptococcus spp. 

What is Coliform? 

Coliform micro organism means aerobic and facultative anaerobic, gram negative non sporeforming rod shaped micro organisms that can ferment sugar (lactose in milk) into acid (lactic acid) and gas at 30-35 C within 48 hrs. 
e.g. Escherichia coli, Enterobacter and Klebsiella. 

How to remember Gram staining Procedure?

Gram staining is nothing but to differentiate gram positive bacteria and gram negative bacteria. Students should remember the phrase “CLAS”

C = Crystal violet, L = Lugol’s iodine, A = Alcohol, S = Saffranin . The sequence of staining to memorise. Please see the details about Gram staining procedure Here

Gram Positive Rods

Gram Negative Rods

Pathogenic Microorganisms in raw milk

In raw milk Pathogenic micro organisms comes from infected udder, diseased condition of animals or from infected human handlers. Pathogens may come from faecal or non faecal sources also. 

List of pathogens which may present in raw milk depending on hygienic and environmental condition. 

1. Sreptococcus agalactiae, Staphylococcus aureus, E. coli (Bovine mastitis – which is a udder disease of cow). Its frequent

Udder with mastitis infection

2. Listeria monocytogenes, Bacillus cereus, Nocardia spp. Coxiella burnetii, Actinomyces spp., Clostridium perfringens,  (Bovine mastitis – udder disease). Its not frequent. 

3. Brucella spp. such as Br. melitensis, Br. abortus, Br. suis (Brucellosis in milch animals).

4. Microbacterium tuberculosis, Microbacterium bovis (Tuberculosis or TB in milch animals). 

5. Streptococcus pyogenes (contamination from human handlers). 

6. Salmonella typhi, Salmonella paratyphi, Vibrio clorerae, Shigella spp., such as Shigella flexneri, Shigella dysenteriae (Faecal contamination, infected human as carrier). 

7. Streptobacillus moniliformis (insects and rodents are carrier). 

8. Yeast suach as Cryptococcus neoformans (infrequent bovine mastitis). 

9. Viruses
Foot and Mouth disease virus or FMD virus (contamination from infected milch animals), Tick-borne encephalitis virus (milch animals infected with tick bites), Adenovirus (contamination from human handlers), Enterovirus (human handlers), Hepatitis virus (Human – infected or as carrier). 

[Note: All Microorganisms should read/write as italicized, e.g. Corynebacterium diphtheriae. first letter of genus, C=Capital letter; first letter of species, d=small letter]

Note: All bacteria are microorganism but all microorganisms are not called as bacteria.

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