Studies on coagulase-negative staphylococci in patients undergoing cardiac surgery. Different hypotheses for the origin of multiply-resistant isolates.

Hypotheses for the origin of multiply-resistant coagulase-negative staphylococci from 146 patients undergoing cardiac surgery were tested. All received cephalothin per-operatively. Antibiotic susceptibility testing, phage-typing, bio-typing, and test for Tween-80-splitting enzyme were used to characterize 132 isolates from nose swabs. Seventy-five percent of the pre-operative susceptible isolates were of biotype 1, while biotypes 3 and 4 made up 59% of the post-operative, multiply-resistant isolates. Fifty-three percent of the isolates were typable by phage-typing. Typability of isolates of biotype 1 was high (56%) while almost 75% of biotype 4 were untypable. Susceptible isolates were more often typable than multiply-resistant ones. Of the 146 patients, 105 (72%) carried coagulase-negative staphylococci pre-operatively, only two of them carried multiply-resistant strains. Fifty-nine patients (41%) were colonized with multiply-resistant coagulase-negative staphylococci post-operatively. By combining the results of bio-typing, phage-typing, and test for Tween-splitting enzyme the study made it probable that a maximum of ten patients (6.8%) already carried multiply-resistant strains on admission to the hospital or were carriers of initially susceptible strains which developed multiple-resistance during administration of antibiotics. It therefore seemed likely that most of the patients were deprived of their natural bacterial flora by antibiotic treatment and subsequently colonized post-operatively with multiply-resistant coagulase-negative staphylococci from the environment.