Airborne microbial contamination can impact human health in multiple ways.
Infection, allergic reaction, inflammation, and respiratory disease can result from inhaling microbial aerosols.
Exposure to airborne microorganisms, viruses, and fungi in the built environment may have both positive and negative effects on human health. The presence of bioaerosols has been linked to a variety of human diseases, including infectious/respiratory symptoms, cardiovascular symptoms, and neurological symptoms. As inhaled microorganisms are deposited directly on the moist respiratory tract surfaces, the lung is more susceptible to infection than the gastrointestinal tract.
The inflammatory response and allergic reactivity can be debilitating to sensitised individuals in indoor and outdoor environments, and it has been demonstrated following exposure to airborne bacterial fragments present in airborne dust. Nosocomial infections by airborne pathogens and with antibiotic-resistant bacteria in hospital environments are transmitted among patients.
Overall, airborne microbial contamination can cause respiratory infections, allergies, asthma, hypersensitivity pneumonitis, and other adverse health consequences.
It is therefore crucial to know how many viable colonies are in the air for multiple reasons.
- Microorganisms are difficult to count directly; therefore, colony enumeration is used to determine the concentration of microorganisms in a sample.
- In order for air sampling results to be meaningful, they must be compared with those obtained from other defined areas, conditions, or time periods, and they must always include controls from outdoor air or non-suspicious interior areas.
- The survival of microorganisms during air sampling is inversely proportional to the rate at which air is drawn into the sampler, and the use of a settle plate increases the likelihood of optimal survival of collected organisms.
What is the settle plate method and why is it important?
The settle plate method is a method for testing the air for microbial contamination. It is one of the oldest methods for testing the air for microbial contamination, and it is still deemed beneficial.
The technique involves exposing a Petri dish containing culture media to the air for a predetermined amount of time, allowing biological particles to settle onto the agar surface. The number of microorganisms deposited on the agar surface of the plate during the exposure period is determined by incubating the agar plates at a specific temperature for a specific number of days and tallying the colonies that develop.
The results can be expressed in terms of colony-forming units (CFUs) per unit period. The enumerated colonies can then be classified by genus and species.
In passive air monitoring, settle plates are exposed to the air for a predetermined amount of time and then incubated to enable visible colonies to form and be counted.
Settle plates are used in industries such as food, pharmaceutical, and cosmetics to estimate the number of microorganisms likely to deposit on a product or surface in a given time period.
There have been several publications in the recent literature that demonstrate the utility of settle plates.
In reference  common surgical consequences include surgical site infections (SSIs) was examined. This study looked at microorganisms in the air, on reusable surgical instruments, and on the outside surface of sterile packaging systems used and reprocessed in the operating room (OR) and Central Sterile Supply Department (CSSD). Active air sampling and settling plates measured airborne microbes. Particle counters monitored airborne particle load. Contact agar plates measured microbial load on surgical tools and sterile packaging systems. The OR had the greatest average microbial and particle load (active air sampling: 56 CFU/m3; settle plates: 9 CFU; 0.3 μm particles: 1,958,403 no./m3). OR surgical instruments had no microbial burden (0 CFU). Sterile packaging systems had a maximum microbial load of 64 CFU on their outside surfaces. Coagulation-negative staphylococci were most frequent. The main source of microbial contamination, especially in the OR, is air, not well reprocessed reusable surgical equipment or sterile packing systems.
In reference  Charnley found that ultraclean air prevented deep illness by lowering airborne germs. MRC trials showed efficacy, but registry data have not. The scientists checked theatre air quality since they treat rheumatoid arthritis patients.
Methods: In phase 1, they evaluated air quality utilising settling plates on joint replacement theatre instrument carts for one hour after the incision. Phase 1 scrubs did not wear body exhaust systems. In phase 2, all three employees employed a body exhaust system and carefully positioned surgical lights and trolleys.
Results: In phase 1, the ultraclean zone grew 0.24 colonies/plate/hour, similar to the Charnley trial. The second phase yielded 0.03 colonies/plate/hour (p<0.001). When plates were placed on trolleys in controlled placements, colonies tended to develop on the trolley corners near the clean zone (NS).
Discussion: Phase 1 colony counts matched Charnley investigations. With body exhausts, modern teams may achieve 0.03 colonies/plate/hour. The use of settle plates is encouraged.
In reference  controlling for operating theatre (OT) infections is crucial. Infections can be detected and controlled via microbiological surveillance. This study examined the prevalence of microorganisms in OTs, their types, and OT air and surface contamination. Methods Soap and water cleaned OTs. All surfaces were disinfected and fumigated using quaternary ammonium chemicals. Overnight, OTs closed. They were opened and samples gathered in the morning. The settle plate and swab methods were employed for air and surface sampling, respectively. The institution's microbiological laboratory received samples from four OT surfaces—floor, wall, table, and light—and OT air. Result 487 (29.7%) of 1640 swab samples from eight OTs were positive for bacteria. Aerobic spore-forming Bacilli and Micrococcus constituted the majority. 82 positive cultures out of 200 were from septic OT. The most prevalent isolates from surface sampling of various OTs were aerobic spore-forming Bacilli (221/487), coagulase-negative Staphylococci (74/487), and Micrococcus (67/487). Air bioload was highest in general surgery, septic, and emergency OTs (97, 93, and 91 CFU/M3, respectively). Conclusion When patient load was high, septic and general surgery OTs were most polluted in surface sampling. Pathogenic bacteria were most prevalent on OT walls and tables. The average air bioload of all OTs was 79–97 CFU/M3.
In conclusion, airborne microorganisms, viruses, and fungi can negatively affect human health. The settle plate method is useful to test the air for microbial contamination by exposing a Petri dish containing culture media to the air for a predetermined amount of time (45 minutes). The number of microorganisms deposited on the agar surface of the plate is determined by counting the colonies that develop after incubating the agar plates at a specific temperature for a specific number of days.
You can use this method to determine if indoor mould is or is not a problem for you and your family. Lok at what one of our test plates revealed about someone's bedroom! Look at all those colonies of Penicillium chrysogenum and Candida sp. Once you know what's in your air, you can look for causes and then take action.
1. Dreikausen L, Blender B, Trifunovic-Koenig M, Salm F, Bushuven S, Gerber B, Henke M. Analysis of microbial contamination during use and reprocessing of surgical instruments and sterile packaging systems. PLoS One. 2023 Jan 20;18(1):e0280595. doi: 10.1371/journal.pone.0280595. PMID: 36668667; PMCID: PMC9858816. https://doi.org/10.1371/journal.pone.0280595
2. Thomas AM, Wilkinson M, Garvey MI. Changes in orthopaedic operating theatre practice, monitored using settle plates. Ann R Coll Surg Engl. 2022 Sep;104(8):600-604. doi: 10.1308/rcsann.2021.0328. Epub 2022 Apr 20. PMID: 35442847; PMCID: PMC9433170. https://doi.org/10.1308/rcsann.2021.0328
3. Shukla A, Srivastava S, Srivastava A, Srivastava T. Surveillance of Microbiological Environment of Operation Theaters. Cureus. 2021 Dec 20;13(12):e20525. doi: 10.7759/cureus.20525. PMID: 35070559; PMCID: PMC8765562. https://www.cureus.com/articles/78285-surveillance-of-microbiological-environment-of-operation-theaters#!/