Identifying Pathogens in Milk: Staphylococci and Salmonella

Every batch of milk that reaches consumers has passed through a series of rigorous microbiological checks-and among the most critical of these are tests for Staphylococci and Salmonella. These two pathogens are responsible for some of the most common and serious dairy-related foodborne illnesses worldwide. What makes them particularly challenging is that they can be present in milk without altering its taste, smell, or appearance-meaning laboratory detection is the only reliable way to confirm their absence. Understanding how these pathogens are identified is fundamental to dairy quality assurance.

Why Staphylococci and Salmonella matter in dairy safety 🔗

Milk is an ideal growth medium for bacteria due to its high water activity, neutral pH, and rich nutrient content. Both Staphylococci and Salmonella exploit these conditions, but in different ways and through different contamination routes. According to research published in Toxins, Staphylococcal food poisoning (SFP) is an intoxication that results from consuming preformed enterotoxins in food, with symptoms including nausea, violent vomiting, and abdominal cramping typically appearing within 2-8 hours of ingestion. ScienceDirect notes that staphylococcal food poisoning is among the most common causes of reported foodborne illness in the United States, with dairy products frequently implicated in outbreaks.

Salmonella, on the other hand, poses a different kind of threat. A comprehensive review published in the Journal of Umm Al-Qura University for Applied Sciences identifies Salmonella spp. as a leading cause of foodborne outbreaks worldwide, with contaminated milk and milk products serving as recognized transmission vehicles. Both pathogens are subject to national and international microbiological standards, and their detection in dairy products triggers mandatory corrective action.

Detecting Staphylococci in milk 🔗

Not all Staphylococci are harmful. The dangerous strains-most notably Staphylococcus aureus-are distinguished from harmless species by their ability to produce enterotoxins and express the enzyme coagulase. Detection therefore involves two sequential steps: isolating the organism on selective media and then confirming pathogenicity through confirmatory testing.

Isolation on Baird Parker Agar 🔗

The primary selective medium for Staphylococci in dairy samples is Baird Parker Agar (BPA). Sigma-Aldrich’s technical documentation explains that BPA contains lithium chloride and potassium tellurite, which inhibit most contaminating microflora, while glycine and pyruvate selectively enhance Staphylococci growth. When Staphylococci reduce tellurite to telluride, they produce the characteristic grey-to-black colony coloration the medium is known for.

Milk samples are diluted and spread onto BPA plates, then incubated at 37°C for 24 to 48 hours. Presumptive S. aureus colonies appear as small, black, convex colonies surrounded by a clear halo-the result of a lecithinase (lipolytic) reaction with egg yolk emulsion added to the medium. Baird-Parker agar is now widely recommended by national and international bodies for the isolation and enumeration of coagulase-positive staphylococci in food and animal feedstuffs, and its use is standardized under ISO 6888-1.

The coagulase test: confirming pathogenicity 🔗

Because grey-black colonies on BPA are only presumptive evidence, a coagulase test is required for confirmation. The coagulase test detects the enzyme coagulase, which S. aureus uses to convert fibrinogen in blood plasma into fibrin clots. In practice, suspected colonies are emulsified in rabbit plasma; a positive result-clot formation within hours-confirms the presence of coagulase-positive, potentially enterotoxin-producing staphylococci.

A study published in Food Microbiology evaluated Baird Parker agar and Rabbit Plasma Fibrinogen agar for enumerating coagulase and thermonuclease-positive Staphylococcus in raw milk and soft cheese, highlighting that coagulase and thermonuclease production are the key characteristics used in microbiological analyses for controlling these organisms. Research published in the Canadian Journal of Veterinary Research further demonstrated that modified BPA achieves a sensitivity of 94.8% and specificity of 100% for detecting S. aureus in bulk tank milk, confirming it as the most reliable culture-based tool available.

The heat-stable toxin problem 🔗

One of the most serious challenges with Staphylococci in dairy products is that standard pasteurization kills the bacteria but not the toxins they have already produced. The U.S. FDA’s Bacteriological Analytical Manual states that staphylococcal enterotoxins are heat stable and are not denatured unless exposed to autoclave conditions (121°C at 15 PSI for 60 minutes)-far beyond routine pasteurization parameters. A study in the Journal of Dairy Science confirmed that staphylococcal enterotoxins produced before heat treatment may persist in foods even after standard heat processing. This means that dairy safety depends on preventing contamination and limiting bacterial growth before processing, not just on heat treatment alone.

Detecting Salmonella in milk 🔗

Salmonella detection in dairy follows a multi-stage protocol because the pathogen may be present in very low numbers-sometimes too few to detect by direct plating. The internationally recognized standard is ISO 6579-1:2017, which specifies the horizontal method for detecting Salmonella spp. in food products including milk and milk products.

Pre-enrichment and selective enrichment 🔗

The first step in Salmonella detection is pre-enrichment in a non-selective broth-typically Buffered Peptone Water (BPW)-which allows any sublethally injured Salmonella cells to recover before selective pressure is applied. Sigma-Aldrich’s technical guidance on Salmonella detection notes that following pre-enrichment, selective enrichment is performed using broths such as Rappaport-Vassiliadis Soy (RVS) broth or Muller-Kauffmann Tetrathionate-Novobiocin (MKTTn) broth, which suppress competing organisms while allowing Salmonella to multiply. The same review in the Journal of Umm Al-Qura University for Applied Sciences explains that MKTTn broth has become the second preferred selective enrichment medium in ISO 6579 due to its improved selectivity through the addition of novobiocin to suppress Proteus spp.

The ISO standard also specifically notes that in dried milk products and cheese, Salmonella may be sublethally injured, and that the selective enrichment step should be extended by an additional 24 hours for such products to improve recovery.

Plating on differential media: XLD agar 🔗

After enrichment, samples are plated onto selective differential media. Xylose Lysine Deoxycholate (XLD) agar is the primary isolation medium specified by ISO 6579-1. XLD agar works on a dual mechanism: sodium deoxycholate inhibits Gram-positive bacteria, while Salmonella, after exhausting xylose, decarboxylate lysine via lysine decarboxylase, raising the medium’s pH and producing colonies that remain red or develop characteristic black centres from hydrogen sulfide production. The ISO 6579-1:2017 standard specifies that typical Salmonella colonies on XLD agar have a black centre with a lightly transparent reddish zone, and plates are incubated inverted at 37°C for 24 ± 3 hours before examination.

A study published in the International Journal of Food Microbiology evaluating enrichment broths and plating media for Salmonella isolation from dairy products found that XLD agar with Rappaport-Vassiliadis (RV) enrichment achieved 100% sensitivity and 93.71% specificity, confirming it as an effective approach for dairy sample testing.

Confirmation with Triple Sugar Iron (TSI) agar 🔗

Presumptive Salmonella colonies from XLD agar are transferred to Triple Sugar Iron (TSI) agar for biochemical confirmation. TSI is a tube-based medium containing three sugars-glucose (0.1%), lactose (1%), and sucrose (1%)-along with the pH indicator phenol red and sodium thiosulfate for hydrogen sulfide detection. According to Microbiology Info, the medium is inoculated by stabbing the butt and streaking the slant, then incubated at 35-37°C for 18-24 hours. The characteristic Salmonella reaction on TSI is an alkaline slant over an acid butt (red slant / yellow butt), often accompanied by blackening in the butt due to hydrogen sulfide production.

The U.S. FDA’s Bacteriological Analytical Manual for Salmonella makes clear that TSI cultures failing to give this typical alkaline/acid pattern should be considered non-Salmonella, while cultures showing the characteristic reaction are retained for further biochemical and serological confirmation tests. Final identification may also include serotyping using polyvalent somatic (O) antisera to establish the specific serovar of Salmonella present.

Why pathogen detection in milk cannot be skipped 🔗

The cumulative purpose of all these detection steps is to ensure that dairy products are safe before they reach consumers. Both Staphylococci and Salmonella present hidden hazards: they can be present in seemingly normal-looking milk, and their effects range from rapid-onset intoxication (Staphylococci enterotoxins) to invasive salmonellosis requiring hospitalization. The CDC has reported that staphylococcal enterotoxin A can cause illness at ingested amounts as small as 20 nanograms, emphasizing how even trace contamination can have serious consequences.

Systematic pathogen detection also underpins compliance with regulatory frameworks. Sigma-Aldrich’s regulatory compliance guidance notes that the updated ISO 6579-1:2017 standard now incorporates milk and milk products as well as primary production samples, reflecting growing international consensus on the need for comprehensive dairy pathogen testing. Dairy processors that follow these standardized protocols-from selective media to biochemical confirmation-create a documented evidence trail that satisfies both national food safety authorities and international trade requirements.

Beyond compliance, consistent detection programs make it possible to trace contamination back to its source-whether infected cow udders, personnel hygiene lapses, or equipment sanitation failures-enabling targeted corrective action before a contamination event escalates into a public health incident.

What do you think? Given that staphylococcal enterotoxins can survive standard pasteurization, should regulatory standards require toxin testing in addition to bacterial count limits for all commercially processed dairy products? And with multi-step detection protocols involving pre-enrichment, selective plating, and biochemical confirmation, what challenges might small-scale dairy producers face in meeting these international microbiological standards?

References

1. https://pmc.ncbi.nlm.nih.gov/articles/PMC3153270/
2. https://www.sciencedirect.com/topics/medicine-and-dentistry/staphylococcal-food-poisoning
3. https://link.springer.com/article/10.1007/s43994-024-00205-2
4. https://www.sigmaaldrich.com/US/en/technical-documents/technical-article/food-and-beverage-testing-and-manufacturing/microbiological-analysis-for-food-and-beverage/detection-and-differentiation
5. https://en.wikipedia.org/wiki/Baird-Parker_agar
6. https://www.iso.org/standard/23036.html
7. https://pubmed.ncbi.nlm.nih.gov/20417392/
8. https://pmc.ncbi.nlm.nih.gov/articles/PMC1687136/
9. https://www.fda.gov/media/183699/download
10. https://www.journalofdairyscience.org/article/S0022-0302(16)30616-6/pdf
11. https://www.iso.org/standard/56712.html
12. https://www.sigmaaldrich.com/US/en/technical-documents/technical-article/microbiological-testing/pathogen-and-spoilage-testing/salmonella-contamination-and-testing
13. https://www.innovationdiagnostics.com/en/dehydrated-culture-media/1366-xld-agar-iso-6579-1-.html
14. https://cdn.standards.iteh.ai/samples/56712/37da386eff674e07b35f9025371ee283/ISO-6579-1-2017.pdf
15. https://pubmed.ncbi.nlm.nih.gov/18992207/
16. https://microbiologyinfo.com/triple-sugar-iron-tsi-test/
17. https://www.fda.gov/media/79991/download
18. https://www.cdc.gov/mmwr/preview/mmwrhtml/mm6250a2.htm
19. https://www.sigmaaldrich.com/US/en/technical-documents/protocol/food-and-beverage-testing-and-manufacturing/regulatory-compliance-for-food-and-beverage/iso-6579-1-salmonella-detection