The most important resistance mechanism to beta -lactam antibiotics is the plasmid- mediated beta -lactamase and the common criterion for the epidemiology of these enzymes is the determination of their biochemical characteristics. Surveys of plasmid- encoded beta-lactamases of Gram -negative bacteria used to investigate their relative clinical importance have been poorly performed and rarely conducted outside the developed world.A Survey of uropathogenic strains and of Salmonellae and Shigellae, isolated in South India in 1984 revealed a higher incidence of ampicillin resistance (Minimum Inhibitory Concentration [MIC] >10mg/1) than had ever been reported before (Enterobacteriaceae 80.9%, Salmonellae 90 %, Shigellae 66 %). Only the enterobacterial strains showed any significant resistance to the first generation cephalosporin, cephaloridine (MIC >10mg/1). However, 65.7% of the Salmonella, strains were cefuroxime resistant. Cefuroxime resistance in these Salmonella, strains was accompanied by the widespread distribution of the novel OXA -El like enzyme with more hydrolytic activity against the drug, even though other resistance mechanisms may play subsidiary role. A small proportion of all species conferred resistance to third generation cephalosporins. In the individual species, there was a very high incidence of ampicillin resistance (E.coli 77 %; Klebsiella 69 %) and cephaloridine resistance (E.coli 57 %; Klebsiella 96 %). Many of the ampicillin resistant strains harboured either auto -transferable or mobilisable plasmids (41.8 %). Characterisation of the plasmid DNA from the E.coli transconjugants revealed the existence of 37 different plasmids types. The transconjugants from Klebsiella, Salmonella and Shigella possessed fewer plasmids types than those from E.coli. Most plasmids possessed resistance genes to aminoglycosides and to six or more ii
drugs. Beta -lactamase studies revealed that TEM -1 was the most predominant enzyme in all transconjugant strains followed by OXA -1, SHV -1, TEM -2, OXA -2 and the novel enzyme SAR -2. The SAR -2 enzyme was fully characterised and had a higher pI (8.3) than any previously characterised plasmid- mediated beta -lactamase. It had a broad -spectrum activity with the molecular weight of 36000. In addition, the unusual observations of E.coli strains producing both the PSE -1 and PSE -2 beta - lactamases and strains hyperproducing the TEM -1 were made and these strains were studied further.The development and mechanisms of resistance to beta -lactam/beta -lactamase inhibitor combinations (ampicillin and clavulanic acid) have been performed with laboratory strains possessing the ampicillin resistance plasmids R1, R1010 and extended -broad spectrum beta -lactam resistance plasmids. The results show that challenge with clavulanic acid alone did not affect the expression or integrity of the beta -lactamase whereas challenge with the combination of ampicillin and clavulanic acid caused radical changes - i the expression of the beta -lactamase. In some cases there were multiple copies of genes which resulted in hyperproduction of TEM -1 enzyme and this was sufficient to resist the combinations. Similarly, elevated SHV -1 enzyme production resulted in a resistance mechanism with no concurrent reassortment of the plasmid R1010. Unfortunately, these variants also conferred resistance to second and third generation cephalosporins. The enzymes TEM -3, TEM -5 and TEM -7 were not hyperproduced but the corresponding strains were stable against the combination at higher inoculum density. Importantly, the SHV -1 beta -lactamase had become active against cefotaxime as it was able to hydrolyse the drug. Evidence of this type of resistance (hyperproduced TEM -1 and SHV -1 enzymes) to clavulanic acid is now emerging in clinical practice
