No abstract

Sixty-four ampicillin-resistant strains of Escherichia coli were studied. Six characters were examined: (i) resistance to ampicillin, cephalothin, and carbenicillin, (ii) synergy between ampicillin and cloxacillin, (iii) level of,-lactamase activity after osmotic shock, (iv) transferability of ampicillin resistance, (v) immunological characterization of the enzyme, and (vi) determination of substrate profiles. One class of strains was found in which synthesis of ,B-lactamase is inferred to be plasmid mediated; these strains are highly resistant to ampicillin and carbenicillin, sensitive to cephalothin, do not show synergism between ampicillin and cloxacillin, and reveal a high enzymatic activity after osmotic shock. A second class is formed by strains for which ,-lactamase synthesis is inferred to be chromosomal; these strains present a low resistance level to ampicillin, are sensitive to carbenicillin and resistant to cephalothin, show a synergism between ampicillin and cloxacillin, and reveal a very low enzymatic activity after osmotic shock. These characters may be used to differentiate periplasmic and cell-bound /-lactamases.

A strain ofS. typhimuriumcarrying transferable determinants, one for resistance to ampicillin (A), another for resistance to streptomycin and sulphonamides (SSu), was irradiated with ultraviolet light. A clone resulting from this treatment had lost streptomycin resistance and now carried the A determinant and a new determinant, ASu. Except for coding for ampicillin resistance, the ASu determinant was homologous with SSu. The A moiety of ASu produced ampicillin (penicillin) resistance of the same degree as the original A determinant. It was therefore concluded that irradiation had resulted in the elimination of the S gene in the SSu determinant and its replacement by an A gene to form ASu. Experiments with the ASu and SSu determinants suggest that there is normally only one copy of a resistance determinant in the host cell and only one cell site into which it can integrate.

A selection of fi+ resistance factors and transfer factors, when introduced into K12F+, showed a range of inhibitory activity of lysis by the male-specific phage ju, 2. This range can be used to subdivide the fi+ factors into fi +1 , fi +z , fi +3 and fi +i classes, according to the degree of inhibition of fi 2 lysis. To this subdivision can be added restriction of the 'femalespecific ' phage 0 2.Introduction of all the fi + factors tested into K12HfrH totally inhibited lysis by /t 2 in spot tests, but with two fi +1 and one fi+ 2 factors visible lysis was obtained in agar-layer tests. These three factors caused least inhibition of transfer of pro by HfrH. It can be assumed that both, tests reflect lower inhibition of sex fimbrial formation by these fi + factors than by the remainder.The^~ factors, when introduced into K12, can be subdivided on the basis of restrictive effects on phage 2.These effects can be added to phage restriction in the salmonellae for the purposes of further classification of the transfer factors and It-factors.