Why do we need sanitary-indicator microorganisms (SIMs)?
Environmental objects are tested for the presence of pathogenic, opportunistic, and sanitary-indicator microorganisms. Searching for pathogens directly is difficult, so microbiologists use an indicator system — SIMs.
Their detection indicates that the object is contaminated with human or animal excretions. The biological rationale is simple: SIMs inhabit the same biotopes (organs) as infection agents and are excreted into the environment via similar pathways. If Escherichia coli is present in water, it means fecal contamination has occurred, and with a high degree of probability, intestinal infection agents may also be present.
An ideal SIM must meet strict criteria: inhabit only the host organism, not multiply in water and soil, survive in the environment no worse than pathogens, outnumber them numerically, and be easily detected by simple laboratory methods.
Metrology: how microbial contamination is measured
To assess the sanitary condition, two key criteria are used: the qualitative composition of SIMs and the total microbial load. Laboratories operate with the following concepts:
- Total microbial count (TMC) is the total number of bacteria per unit of mass or volume. The main parameter here is TVC (mesophilic aerobic and facultatively anaerobic microorganisms). They are counted by visible colonies grown on solid media at 37 °C for 24 hours.
- Index is the number of indicator bacteria in a specific volume (for example, the coliform index in 1 liter of water).
- Titer is the smallest volume (or mass) of a substrate in which at least one cell of the target microbe can be detected.
Environmental object control
The set of controlled microorganisms is strictly tied to the type of object being studied.
Water and soil. The main marker is fecal contamination. Coliform bacteria and enterococci are sought. If thermotolerant coliforms growing at 44 °C are detected, the contamination is recent. If Clostridium perfringens spores predominate, it is old. According to standards, the TMC in drinking water must not exceed 50 CFU/ml, and coliform bacteria must be completely absent in 100 ml.
Indoor air. Samples are taken using natural sedimentation (onto Petri dishes) or aspirators. The air is checked for upper respiratory tract microflora — Staphylococcus aureus (Staphylococcus aureus) and hemolytic streptococci. In operating rooms, requirements are maximal: Staphylococcus aureus is not allowed at all, and the TMC must not exceed 100 CFU/m³.
Household items (in healthcare facilities, public catering). Washings are taken from hands, equipment, and tableware. The goal is to find coliforms, Staphylococcus aureus, enterococci, and total contamination. The presence of gram-negative bacteria may indicate a general unsanitary condition and dampness.
Evaluation of food products and medications
Products are tested routinely and on epidemiological indications. Interestingly, standard TMC norms are not applicable to fermented milk products (kefir, cottage cheese) because they initially contain a huge biomass of lactic acid bacteria. In kefir and koumiss, even the presence of yeasts is considered normal due to combined fermentation.
In pharmacy, sanitary microbiology controls medications (medicinal products). Moisture in herbal raw materials or production defects lead to microbial proliferation, which changes the properties of the drug. Medications are divided into:
- Sterile (injections, eye drops): microbes should not be present at all (controlled by inoculation into thioglycolate medium and Sabouraud medium).
- Non-sterile (tablets, ointments): a limited number of cells is allowed, but the presence of Enterobacteriaceae, Pseudomonas aeruginosa, and S. aureus is strictly prohibited.
Briefly: answers to topical questions
What methods and devices are used for air sampling for microbiological analysis?
Sedimentation, aspiration, and filtration methods are used for air sampling.
- Sedimentation method (Koch's method) is the passive settling of microbes onto the surface of an open Petri dish with nutrient medium.
- Aspiration method (forced sedimentation) is the precipitation of microbes from an air stream using impactors (Krotov apparatus, Typhoon-R40M samplers, Bio Sas Super, Mas 100) that direct the stream onto solid medium.
- Filtration method is passing air through a liquid nutrient medium using impingers (Rechmensky, POV-1, Dyakonov devices).
Which pathogenic bacteria are most commonly transmitted through contaminated water?
Pathogens of infections with a fecal-oral transmission mechanism, as well as some other pathogens, are transmitted through contaminated water. The main pathogenic bacteria detected during sanitary and microbiological water testing include:
- Enterobacteria — causative agents of intestinal infections.
- Cholera vibrios (Vibrio cholerae).
- Anthrax agents (Bacillus anthracis).
- Botulism agents (Clostridium botulinum).
- Legionellosis agents.
What sanitary-indicator microorganisms are used to control the quality of meat and meat products?
During routine sanitary and microbiological control of food products, including meat and meat products, markers of fecal or other contamination are detected. The main sanitary-indicator microorganisms used are:
- Escherichia coli (Escherichia coli).
- Coliform bacteria.
- Enterococci.
- Staphylococcus aureus (Staphylococcus aureus).
- Proteus (Proteus).
- Sulfite-reducing clostridia.
Additionally, meat analysis involves targeted screening for pathogens, particularly Salmonella species.
Which genera of microorganisms are included in the coliform group (coliform bacteria)?
The coliform group (coliform bacteria), which serve as primary indicators of fecal contamination and belong to coliforms, includes the following genera:
- Escherichia
- Citrobacter
- Enterobacter
- Klebsiella
These microorganisms are Gram-negative, oxidase-negative bacilli of the family Enterobacteriaceae capable of fermenting lactose to acid and gas at 37 °C.
To remember the markers of fecal contamination, rely on "heat and resilience": Fresh means thermotolerant coliforms (still retaining intestinal warmth, growing at 44 °C). Long-standing means clostridia (leaving behind resilient Clostridium perfringens spores when everything else has already perished).
Class and exam questions
Why is the number of spore-forming bacteria determined in the air?
In indoor air, spore-forming bacteria serve as a marker of contamination by soil particles and street dust.
What is the special feature of controlling antimicrobial drugs?
To test them for sterility, the membrane filtration method is used: the sample is passed through a filter, then the filter is washed free of antibiotics and only then placed into a nutrient medium.
What does the detection of thermophiles in soil signify?
Thermophiles (bacteria capable of growing at 60 °C) in soil serve as an indicator of manure or compost contamination.
What else to study on this topic
- Biochemical and cultural properties of total coliforms (TCC)
- Quality requirements for canned products and botulinum toxin control
- Specifics of sanitary-microbiological analysis of toothpastes and elixirs
- Design and operating principles of impactors and impingers for air sampling
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