Frequency of plasmid-determined beta-lactamases in 680 consecutively isolated strains ofEnterobacteriaceae

TLDR: The frequency of Enterobacteriaceae strains which produce plasmid-mediated beta-lactamases is reported and the different types of betalactamase types are specified, somewhat higher than reported in the literature.

Abstract: Sir, We report on the frequency of Enterobacteriaceae strains which produce plasmid-mediated beta-lactamases and specify the different types of betalactamases. Six hundred and eighty Enterobacteriaceae were consecutively isolated from human pathological specimens by three clinical bacteriological laboratories in 1981-1982. All strains were identified by standard procedures. The MICs of arnpiciUin, amoxiciUin plus clavulanic acid (ratio 2 : 1) and eephalothin were determined for these strains on Mueller-Hinton agar using an inoculum of about 10 s CFU. According to a procedure published elsewhere (1), nitroeephin was added to disrupted bacteria to detect the presence of enzyme. The hydrolysis spectrum of the strains' overall beta-lactamase activity provided a rough substrate profile. The isoelectric point of the beta-lactamases was studied on poly. acrylamide gels by isoelectrofocusing (2, 3) using sonieated extracts that had been purified by ultrafiltration and concentrated when necessary. The betalactamase types were identified by comparison with standard enzymes (2, 3). Beta.lactamase activity was detected in 588 of the 680 strains: 270 showed cephalosporinase activity, 315 broad-spectrum enzyme activity and three, penieillinase activity. The 92 strains without activity included Salmonella spp. (14) and Proteus mirabilis (73). Three hundred and seventeen strains were susceptible to arnpicfllin; those having cephalosporinase activity alone or none at all were presumed to have only chromosome-mediated beta-lactamase. Three hundred and sixty-three (53.38 % of the total) were ampieillin-resistant (MIC > 16/ag/rnl); this ineluded all strains with broad-spectrum beta-Iactamase activity except one (314), plus 37 with cephalosporinase activity, three with penieillinase and nine without enzymatic activity. The isoeleetric point of the beta4actamases of all ampicillin-resistant strains was determined, in addition to that of four susceptible strains (one Escherichia coli with broad-spectrum activity, one Serratia spp., one Otrobacter spp. and one indole-positive Proteus spp.). Values were obtained for 340 strains (92.64 %); 297 of these showed plasmid-mediated beta-lactamase types and 43 showed only chromosome-mediated enzymes. In agreement with other publications (4, 5), the plasmid-mediated beta-lactamase encountered most frequently was TEM-I (87.54 % of the total strains, including those having one or two enzymes). The second most frequent was SHV-1 (in 14.81% of the total); this frequency is somewhat higher than reported in the literature (4, 5). In 19 strains two different types were identified. Two hundred and twenty-nine strains of Escherichia coli, Salmonella spp., Shighella spp. and Proteus mirabilis showed broad-spectrum beta-lactamase activity and all but one were ampicillin-resistant; 17.8% were resistant (MIC>16/~g/ml) to the combination amoxicillin + clavulanic acid and 30% to cephalothin (MIC > 16/ag/ml). Plasmid-mediated beta-lactamases were identified in 40.83 % of the strains, 97.33 % of which were TEM-1 type. The 57 strains of Klebsiella spp. were resistant to ampicillin, 27.27 % were resistant to amoxicillin + clavulanic acid and 32.8 % to cephalothin; broadspectrum activity was detected in 44 strains. Plasmidmediated beta-lactamases were identified in 73.68 % of these strains, SHV-1 (95.23 %) being the most frequent followed by TEM-1 (30.95 %); two different types were identified in 28.57 % of the strains. The frequency of plasmid-mediated beta-lactamases differs within the genus; whereas only one of 11 Klebsiella oxytoca strains carried plasmid-mediated betalactamases, 41 were observed in 43 Klebsiella pneumoniae strains.