Effect of inoculum size on ampicillin and amoxycillin susceptibility determined by gas-liquid chromatography for members of the family Enterobacteriaceae.

Inoculum size had only a small effect on the results for ampicillin and amoxycillin susceptibility testing of Escherichia coli. In contrast, a difference in inoculum size from 10(6) to 10(8) CFU/ml profoundly affected the results for ampicillin and amoxycillin susceptibility testing of Proteus mirabilis, causing a change from susceptibility to complete resistance. These different effects of inoculum size were observed both when susceptibility was determined by the suppression of a characteristic metabolic product analyzed by head-space gas-liquid chromatography (HS-GLC) and when it was determined by MIC testing in broth. Inoculum size affected the results for ampicillin susceptibility of P. mirabilis determined concurrently with the rapid HS-GLC urine test, because 10(8) CFU/ml may occur in urine specimens. In the rapid test, significant numbers of Escherichia, Klebsiella, Citrobacter, Proteus, Morganella, and Providencia spp. in urine specimens are detected within 4 h by HS-GLC analysis for characteristic metabolic products in cultures. Most P. mirabilis in urine specimens appeared to be ampicillin resistant in the rapid HS-GLC test but were reported to be ampicillin susceptible in hospital laboratory agar dilution tests 2 days later. However, ampicillin susceptibility results for Escherichia, Klebsiella, and Citrobacter spp. agreed with hospital laboratory reports. It was concluded that reports of ampicillin susceptibility or resistance for the commonest cause of urinary tract infections, E. coli, within 4 h of receipt of the specimen would be clinically valuable and that a provisional report of ampicillin resistance for P. mirabilis would not lead to ineffective therapy.