N.60 – Microrganism stress


Effect of Impact stress on microbial recovery on an agar surface of an active air sampler

Microbial stress due to impaction of microorganisms onto an agar collection surface is difficult to experimentally demonstrate. Recovery and injury due to collection depends not only on the magnitude of the impaction velocity, but also on the degree to which the microorganisms may be embedded in the collection medium.


“Different microbial air samplers have different design and operating characteristics. When the nozzle opening in an impactor is very small, the air flow has to turn 90° in a very small space. The impact stream depends on the impaction velocity which is approximately equal to the air velocity in the nozzle. If the nozzle opening is larger and the impaction velocity is the same, the microorganism has more space to turn 90°. Thus, even for the same impaction velocity, in one impactor (with larger holes) the microorganism may come in for a soft landing, while in another impactor (with smaller holes) the microorganism may embed itself in the agar.”

This was the reason why the holes of the aspirating head of TRIO.BAS air samplers are 1 mm in diameter: to have a better soft impaction on agar, that means less impact stress for the microorganism..


Shelny Stewart, Sergey Grinshpun, Klaus Willeke, Silva Terzieva, Vidmantas Ulevictus, Jean Donnelly. Effect of impact on microbial recovery on an agar surface. Applied and Environmental Microbiology, Apr 1995, p. 1232-1239.