THE MOST FREQUENTLY OCCURRING AMINOGLYCOSIDE RESISTANCE MECHANISMS - COMBINED RESULTS OF SURVEYS IN 8 REGIONS OF THE WORLD

Aminoglycoside-resistant isolates from different patients were collect ed in 149 hospitals in eight regions of the world. Aminoglycoside resi stance mechanisms(1) were determined in 11,079 of these isolates by th e correlation of resistance phenotypes to 12 aminoglycosides and DNA h ybridisation with up to 19 resistance genes. A very large diversity of different resistance mechanisms was found. For example, in scherichia -Morganella-Proteus-Salmonella-Shigella, a total of 53 different mecha nisms was found among the 2080 isolates studied. Therefore, the most c ommon resistance mechanisms in the seven different pathogen groups wer e summarised. The seven pathogen groups were chosen so that each one h ad a unique mixture of resistance mechanisms. Among Enterobacteriaceae , the Citrobacter-Enterobacter-Klebsiella, Providencia and Serratia gr oups had a particularly high incidence of combinations of resistance m echanisms especially when compared with data from eight earlier survey s.(2-9) These mechanisms often tended to be combinations of previously common gentamicin-modifying enzymes with AAC(6')-I, an enzyme which c an acetylate tobramycin, netilmicin and amikacin but which does not us ually cause resistance to isepamicin. Resistance rates among the four Enterobacteriaceae groups reflected the incidence of these combination s; that is, amikacin resistance was 43.1%, 21.7%, 4.2% and 37.6% while that to isepamicin was 9.9%, 9.7%, 2.7% and 7.6%, respectively. A ver y large diversity of mechanisms was found in Pseudomonas. However, thr ee mechanisms - permeability, AAC(6')-II and ANT(2'')-I - were very co mmon, both alone as single mechanisms and in combination with each oth er or other mechanisms. Because of the high incidence of permeability resistance in Pseudomonas, resistance to all aminoglycosides was quite high and the difference between amikacin (41.7%) and isepamicin (36.8 %) was smaller. The greatest diversity of resistance mechanisms (67 in 1189 isolates) was found in Acinetabacter. The most common mechanisms - APH(3')-VI and AAC(3)-? - were generally very rare in other genera but found frequently in Acinetobacter in combination with a wide varie ty of other mechanisms. The most frequent resistance mechanisms in Aci netobacter varied by geographic region much more than did mechanisms i n other genera. The resistance rates to all of the clinically useful a minoglycosides were generally higher in Acinetobacter than in any othe r pathogen group. In contrast to the other pathogen groups, only seven resistance mechanisms were found in the 898 resistant Staphylococcus. These were composed of the three known single mechanisms(1) and four combinations of these three mechanisms. Because the principal resistan ce mechanism, APH(2'') + AAC(G'), confers high-level resistance to gen tamicin and tobramycin and a lower level of resistance to amikacin, is epamicin and netilmicin, the observed resistance rate to netilmicin (1 5.0%) was the lowest for any of the aminoglycosides.