The recovery of anaerobic bacteria from swabs in three transport systems. Statistical evaluation by application of a variance component model.

The recovery of six anaerobic species from swabs in three transport systems was quantitatively evaluated. The test strains were Bacteroides fragilis, B. melaninogenicus ss. intermedius, Fusobacterium necrophorum, Peptostreptococcus anaerobius, Eubacterium lentum, and Clostridium ramosum. The transport systems were a charcoal-impregnated cotton swab in modified Stuart's transport medium (MS), or a plain cotton swab in either a dry aerobic tube (DA) or an aerobic agar tube (AA). Swabs were charged with a known number of organisms of the test strain and inserted in the appropriate transport systems. Immediately hereafter the swabs were transferred to glass tubes with reduced buffer. After vigorous shaking, aliquots from the buffer were seeded onto blood agar plates, and the number of colony forming units (CFU) was counted after incubation. The number of CFUs from independent replications had a variation that was too large to be accounted for by the Poisson distribution. The probability of recovery (POR) was defined as the number of CFU recovered from a swab as a percentage of the original inoculum. The logarithms of the POR values were analyzed with a variance component model. This showed that estimated values of POR were significantly higher in DA (44%) than in AA (23%), while the value for MS (33%) did not differ significantly from those for AA or DA. It is concluded that in MS, friction between swab and medium does not contribute significantly to bacterial recovery loss, as compared to AA and DA.