Frequency of disinfectant resistance genes and genetic linkage with beta-lactamase transposon Tn552 among clinical staphylococci

Abstract

A total of 61 strains of Staphylococcus aureus and 177 coagulase-negative staphylococcal strains were isolated from the blood of patients with bloodstream infections and from the skin of both children under cancer treatment and human immunodeficiency virus-positive patients. The MIC analyses revealed that 118 isolates (50%) were resistant to quaternary ammonium compound-based disinfectant benzalkonium chloride (BC). The frequencies of resistance to a range of antibiotics were significantly higher among BC-resistant staphylococci than among BC-sensitive staphylococci. Of 78 BC-resistant staphylococcal isolates, plasmid DNA from 65 (83%), 2 (3%), 43 (55%), and 15 (19%) isolates hybridized to qacA or -B (qacA/B), qacC, blaZ, and tetK probes, respectively. The qacA/B and blaZ probes hybridized to the same plasmid in 19 (24%) staphylococcal strains. The plasmids harboring both qacA/B and blaZ genes varied from approximately 20 to 40 kb. The Staphylococcus epidermidis Fol62 isolate, harboring multiresistance plasmid pMS62, contained qacA/B and blaZ together with tetK. Molecular and genetic studies indicated different structural arrangements of blaZ and qacA/B, including variable intergenic distances and transcriptional directions of the two genes on the same plasmid within the strains. The different organizations may be due to the presence of various genetic elements involved in cointegration, recombination, and rearrangements. These results indicate that qac resistance genes are common and that linkage between resistance to disinfectants and penicillin resistance occurs frequently in clinical isolates in Norway. Moreover, the higher frequency of antibiotic resistance among BC-resistant strains indicates that the presence of either resistance determinant selects for the other during antimicrobial therapy and disinfection in hospitals.

FIG. 1.

Percentages of antibiotic-resistant CNS isolates from patients with bloodstream infections (n = 60) and from the skin of children under cancer treatment (n = 61) among BC-resistant and BC-sensitive isolates. Antibiotics used are PEN, oxacillin (OXA), cephalothin (CEP), cefuroxime (CEF), imipenem (IMI), cotrimoxazole (COT), ERY, clindamycin (CLI), GEN, ciprofloxacin (CIP), doxycycline (DOX), chloramphenicol (CHL), and fusidic acid (FUS). Susceptibility to CEF, IMI, and CHL was tested only among CNS isolates from patients with bloodstream infections (asterisks).

FIG. 2.

Genetic organization of qac and β-lactam resistance genes in staphylococcal isolates Fol24, Fol33, Fol62, Fol89, Fol90, and Fol100. Genes involved in qac and β-lactam resistance are gray and black, respectively. qacA/B encodes QAC resistance; qacR encodes a putative repressor of qacA/B; sin is a putative staphylococcal recombinase gene; p271 encodes a potential ATP-binding protein; p480 encodes a transposase; binR and binL are resolvase-encoding genes; blaZ is a β-lactamase structural gene; blaR and blaI are β-lactamase regulatory genes; IS257 is a staphylococcal insertion sequence. Primers for detection of specific PCR products are indicated for Fol33. Vertical bars and boxes downstream of blaI, TIR of transposon Tn552 and a resolution site (res), respectively.