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Mathematical models have been developed which describe the effect of lowering the water activity on the growth kinetics of Staphylococcus aureus and Salmonella typhimurium. By treating the lag phase and exponential phase kinetics separately predictions can be made on the extent of microbial growth over successive time/temperature cycles. Staph. aureus was far more tolerant than Salm. typhimurium to lowered water activity and under near growth limiting conditions of water activity and temperature was showing lag periods as long as ca 40 d. The maximum lag period observed for Salm. typhimurium was ca 5 d. Under these conditions the predicted generation times for Staph. aureus were 2-3 d and for Salm. typhimurium.

Carbenicillin or ticarcillin were incubated individually with each of the following antibiotics: gentamicin, tobramycin, sisomicin, amikacin. The residual activity of each aminoglycoside in this mixture was assayed enzymatically. Amikacin was inactivated the least of the aminoglycosides. Both penicillins inactivated each aminoglycoside to a similar extent by a degree which varied according to the medium of incubaiton, the least inactivation being seen in pooled human serum and the most in phosphate buffer at pH 7.4.

SYNOPSIS The adenylyltransferase/acetyltransferase methods of gentamicin assay have been evaluated for accuracy, speed, and cost. For a comparable cost of materials the latter method is more accurate than that using the adenylyltransferase enzyme. The acetyltransferase method is much quicker than the adenylyltransferase due to the shorter time necessary for radioactive counting. Sonication is an easier method of enzyme preparation than the previously used osmotic shock technique. The acetyltransferase method is reproducible and there was a very good correlation between it and a microbiological agar-plate diffusion method.The introduction of enzymatic methods for the estimation of gentamicin concentrations present in patients' sera samples may enable a more accurate and reliable service to be offered by hospital laboratories. The original method of Smith, Van Otto, and Smith (1972) measured gentamicin by the formation of (14C)-adenylylated-gentamicin using (14C)-adenosine triphosphate (ATP) in the presence of a specific adenylylation enzyme derived from a gentamicinresistant Escherichia coli strain.This method has recently been evaluated in comparison with the urease method (Noone, Pattison, and Samson, 1971) and the agar diffusion plate method (Reeves, 1972 (Haas and Davies, 1973) which gives scope for greater accuracy than any of the methods yet published.