Why Do We Need Sanitary Indicator Microorganisms (SIMs)?
Environmental objects are routinely tested for pathogenic, opportunistic, and sanitary indicator microorganisms. Direct screening for all pathogens is complex and demanding; therefore, microbiologists rely on a system of indicator organisms (SIMs).
Their detection indicates that an environment has been contaminated by human or animal waste. The biological rationale is simple: SIMs inhabit the same biotopes (niches) as infectious agents and enter the environment through identical routes. If Escherichia coli is detected in water, it signals fecal contamination, carrying a high probability that enteric pathogens may also be present.
An ideal SIM must meet strict criteria: inhabit strictly the host organism, fail to multiply in water and soil, survive in the external environment at least as well as pathogens, outnumber them, and be easily detected using simple laboratory methods.
Metrology: Measuring Microbial Contamination
Evaluating sanitary conditions relies on two key criteria: the qualitative composition of SIMs and the total microbial load. Laboratories utilize the following standard concepts:
- Total Viable Count (TVC) — the total number of bacteria per unit of mass or volume. The primary parameter here is TAMC (Total Aerobic Microbial Count) and TYMC (Total Yeast and Mold Count). These are quantified by counting visible colonies grown on solid media at 37 °C for 24 hours.
- Index — the number of indicator bacteria found in a specific volume (e.g., the coliform index per 1 liter of water).
- Titer — the smallest volume (or mass) of a substrate in which at least one cell of the target microbe can be detected.
Environmental Quality Control
The panel of monitored microorganisms strictly depends on the type of object being tested.
Water and Soil. The primary marker is fecal contamination. Testing targets coliform bacteria and enterococci. The presence of thermotolerant coliforms capable of growing at 44 °C indicates fresh contamination, whereas a predominance of Clostridium perfringens spores points to remote contamination. According to standards, TVC in drinking water must not exceed 50 CFU/mL, and coliforms must be entirely absent in 100 mL.
Indoor Air. Samples are collected using natural sedimentation (Petri dish exposure) or air samplers (aspirators). Air is screened for upper respiratory tract flora, such as Staphylococcus aureus and hemolytic streptococci. Requirements are strictest in surgical operating theaters: Staphylococcus aureus must be completely absent, and TVC must not exceed 100 CFU/m³.
Environmental Surfaces (Healthcare and Food Service). Swabs are taken from hands, equipment, and utensils. The objective is to detect coliforms, Staphylococcus aureus, enterococci, and total bioburden. The presence of gram-negative bacteria may indicate poor hygiene and excessive moisture.
Evaluation of Food Products and Pharmaceuticals
Food products undergo routine testing as well as epidemiological screening. Notably, standard TVC limits do not apply to fermented dairy products (e.g., kefir, cottage cheese) because they inherently contain a massive biomass of lactic acid bacteria. Furthermore, the presence of yeasts in certain fermented drinks is normal due to combined fermentation pathways.
In pharmacy, sanitary microbiology monitors medicinal products (MPs). Improper storage of herbal raw materials or manufacturing flaws can lead to microbial proliferation, altering drug properties. Pharmaceutical preparations are divided into:
- Sterile (injectables, eye drops): microorganisms must be completely absent (verified by inoculation into thioglycolate broth and Sabouraud media).
- Non-sterile (tablets, ointments): a limited bioburden is permitted, but the presence of Enterobacteriaceae, Pseudomonas aeruginosa, and S. aureus is strictly prohibited.