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We used a membrane filter contact technique to pick up and grow bacteria from artificially contaminated surfaces. We were able to recover individual colonyforming units (CFU) of Staphylococcus aureus from a moist agar surface more efficiently with 3-and 5-,m membrane filters than with Rodac plates, velvet pads, velveteen pads, or smaller-pore membrane filters. The effective transfer of bacteria with the 3and 5-gm membrane filters was 0.96 ± 0.04 (standard error of the mean) and 0.99 + 0.04, respectively, as compared to 0.49 ± 0.03 for Rodac plates, 0.09 ± 0.01 velvet pad imprints, 0.05 + 0.01 for velveteen pad imprints, 0.27 ± 0.02 for velvet pad rinses, 0.005 ± 0.001 for velveteen pad rinses, 0.39 ± 0.02 for 0.45gm filters, and 0.85 ± 0.05 for 1.2-gm filters. In addition, the recovery of S. aureus from contaminated bovine muscle surfaces with the 5-,um membrane flter was similar to that of quantitative dilutions of biopsy material and was significantly higher than the recovery from Rodac plates. The 5-,um membrane filters on a paddle recovered 52 ± 5 CFU/cm2 from artificially contaminated bovine skeletal muscle, the quantitative dilutions of biopsy recovered 69 ± 5 CFU/cm2, and the Rodac plate recovered 5 ± 3 CFU/cm2. Sampling of moist surfaces by the membrane filter contact technique is easy to perform and highly efficient; our data suggest that it could be employed for quantitative cultures of clinical surfaces such as surgical wounds or burns.

Disk diffusion susceptibility tests for enterococci are frequently modified by adding 5% sheep blood (SB) to Mueller-Hinton agar; the performance standards from the National Committee for Clinical Laboratory Standards sanction this addition. Susceptibility testing of aminoglycoside antibiotics is not recommended for enterococci; in actual practice, however, some laboratories do include aminoglycoside antibiotics routinely, and others may test upon request or in selected situations. In examining 50 clinical isolates of enterococci, SB-enriched Mueller-Hinton agar frequently gave enlarged zone sizes that falsely indicated susceptibility (72% for gentamicin and tobramycin), with the average increase in zone size being 6.3 and 7.6 mm, respectively. Comparison agar dilution MICs demonstrated uniform resistance, with or without added SB. The effect was shown to be caused by heme in concentrations as low as 0.03 ,ug/ml, which, when combined with aminoglycoside antibiotics, caused a synergistic growth inhibition of the enterococci, resulting in larger aminoglycoside antibiotic zones. We postulate that the heme effect is related to a catalytic cleavage of intracellular H202 and resultant lipid peroxidation. No other organism or antimicrobial agent tested demonstrated a similar effect, although other investigators have shown a similar phenomenon with the broad-spectrum cephalosporins. Because enterococci grow well and give accurate susceptibility results on Mueller-Hinton agar without SB supplementation and because of the spectrum of definable problems with a number of antimicrobial agents, we recommend that enterococci routinely be tested without SB.