Effect of impact stress on microbial recovery on an agar surface

Abstract

Microbial stress due to the impaction of microorganisms onto an agar collection surface was studied experimentally. The relative recovery rates of aerosolized Pseudomonas fluorescens and Micrococcus luteus were determined as a function of the impaction velocity by using a moving agar slide impactor operating over a flow rate range from 3.8 to 40 liters/min yielding impaction velocities from 24 to 250 m/s. As a reference, the sixth stage of the Andersen Six-Stage Viable Particle Sizing Sampler was used at its operating flow rate of 28.3 liters/min (24 m/s). At a collection efficiency of close to 100% for the agar slide impactor, an increase in sampling flow rate and, therefore, in impaction velocity produced a significant decline in the percentage of microorganisms recovered. Conversely, when the collection efficiency was less than 100%, greater recovery and lower injury rates occurred. The highest relative rate of recovery (approximately 51% for P. fluorescens and approximately 62% for M. luteus) was obtained on the complete (Trypticase soy agar) medium at 40 and 24 m/s (6.4 and 3.8 liters/min), respectively. M. luteus demonstrated less damage than P. fluorescens, suggesting the hardy nature of the gram-positive strain versus that of the gram-negative microorganism. Comparison of results from the agar slide and Andersen impactors at the same sampling velocity showed that 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. Impaction velocity, characterized by the sampler's operating flow rate and inlet design, is unique for each sampling device. The resulting impaction stress influences the recovery and injury of collected microorganisms and ultimately affects the measurement data for colony enumeration. This can be one of the most important reasons for variations that occur when using different sampling devices to measure bioaerosols from the same environment.

Incorporating polymerase chain reaction-based identification, population characterization, and quantification of microorganisms into aerosol science: A review

“…Bioaerosol sampling equipment has collection efficiencies less than 100%, and these efficiencies vary significantly with the aerodynamic diameter of airborne particulate matter. Further, conventional collection devices that harvest particulate matter from air introduce significant sampling stresses, which can substantially reduce the viability and/or culturability of a broad range of airborne microorganisms (Buttner et al, 1997;Stewart et al, 1995).…”

Atmospheric Movement of Microorganisms in Clouds of Desert Dust and Implications for Human Health

“…The benefits of these types of samplers are ease of use, portability, cost, assessment of culturable populations of bacteria and fungi per volume of air, and, as in the case of the Andersen 6-STG-type sampler, the ability to determine the numbers of microorganisms associated with various size ranges of airborne particulates. Drawbacks to the use of these impactors are loss of viability due to impact stress, loss of recovery efficiency due to microorganisms not adhering to agar surfaces, low sample volumes due to low flow rates, and low recovery of ultrafine biological matter in the size range of viruses (220). Centrifugation has been used to collect microorganisms on container walls, wet or dry slides, or nutrient agar (petri dishes and strips, etc.)…”

Airborne Microorganisms From Livestock Production Systems and Their Relation to Dust

“…If the particles are living organisms, they may be inactivated during sampling due to impaction stress (Stewart et al, 1995), impingement stress , and/or dehydration stress (Li et al, 1999). Therefore, in order to indicate how well a bioaerosol sampler maintains the microbial viability and prevents cell damage during sampling, the concept of biological efficiency has been introduced (Griffiths and Stewart, 1999).…”