Effect of contamination levels on antimicrobial susceptibilities tested by Autobac I.

Two gram-positive and five gram-negative species of bacteria commonly isolated from clinical specimens were used in 704 different inoculum ratios against 11 antimicrobial agents to study the effect of contamination levels on the susceptibility patterns of mixed cultures. Staphylococcus aureus, Streptococcus faecalis, Klebsiella pneumoniae, Pseudomonas aeruginosa, Escherichia coli, Proteus mirabilis, and Enterobacter cloacae were used in 12 combinations of two each. The pure culture inoculum tubes were prepared according to Autobac I (Pfizer Inc.) instructions. Various amounts from 0.01 to 2.0 ml of one bacterial inoculum were mixed with various amounts of the other bacterial inoculum to form 16 mixtures to low-thymidine eugonic broth. In the majority of cases, susceptibility to antimicrobial agents started to change at 12.5% contamination level. Of 1,936 mixtures with 5% contamination levels, 98.5 to 99.2% exhibited the same susceptibilities as did the pure cultures. These results suggest that generally contamination levels of more than 5% or 70,000 to 150,000 bacteria per ml, are responsible for a significant change in apparent drug susceptibility.