Resistance of Gram-negative Bacilli and Staphylococci from Blood Cultures to Aminoglycoside Antibiotics. Comparison of 3 in vitro Investigations from Austria 1982–1988

Summary A total of 790 blood culture isolates was collected during 3 study periods in 1982/83, 1984/85 and 1987/88. Staphylococci were the most frequent bacteria in the first two periods (56.5% and 63%, respectively). During the last period, E. coli was the most frequent of all species (27%). Differences in the distribution of bacteria between the laboratories were considerable. In one laboratory in Vienna, coagulase-negative staphylococci dominated in all 3 study periods (32%, 33% and 47%). Susceptibility against gentamicin, tobramycin and netilmicin (MIC ≤ 4 mg/1), as well as against amikacin (MIC ≤ 8 mg/1) were determined by a microdilution method. The resistance rates against gentamicin (G) were 25 %, 21 % and 25%, against tobramycin (T) 27%, 19% and 25%, against netilmicin (N) 6%, 4% and 19%, and against amikacin (A), 5%, 2 % and 19%. Most resistant strains were staphylococci (G 26%–41%, T 26%–46%, N 3%–31%, A 3%–36%), whereas gram-negative bacilli were more susceptible (G 12%–14%, T 7%–11%, N 7%–9%, A M–7.5%). The increase of resistance against netilmicin and amikacin in staphylococci was most striking. Nearly all those strains could be attributed to one laboratory in Vienna. Most of them were coagulase-negative staphylococci. In the first study period, the most frequent pattern was resistance against gentamicin and tobramycin, whereas in the last period resistance to all 4 aminoglycosides dominated. The study demonstrates considerable local differences in antibiotic resistance. Monitoring of antibiotic resistance in collaborative studies using standardized methods should be a valuable tool in planning strategies for controlling an epidemic spread of resistance.