The varying sensitivity to antibacterial agents of micro-organisms in pure vs. mixed cultures

In this study the disc sensitivities of five organisms growing in pure cultures (Staphylococcus aureus, beta-hemolytic Streptococcus, Proteus mirabilis, Pseudomonas aeruginosa, and Candida albicans) were first determined against each of seven antibacterial agents (penicillin, streptomycin, gentamicin, kanamycin, silver nitrate, Sulfamylon, and Betadine). Then the sensitivity of each organism growing in combination with one of the others (10 combinations) was tested against each of the same antibacterials. Significantly increased and decreased sensitivities were found in 30 percent of the cultures with decreases largely predominating. Total obliteration of all sensitivity occurred 10 percent of the time. The changes in sensitivity were not distributed randomly but rather were associated more with particular agents and organisms. Sulfamylon, was associated with decreases 70 percent of the time with sensitivity obliteration in 50 percent of the tests. Streptococcus led all the organisms, being associated with decreases in half of the tests. It is possible that mixed-culture sensitivities could provide the most valid information when mixed infections exist, since they more closely simulate the real clinical situation. Therefore it is suggested that both mixed and pure culture sensitivity testing be done for all mixed infections.