Multiresistant strain of Listeria monocytogenes in Greece

Abstract

In 1990 two reports of multiply-resistant Listeria monocytogenes strains and one report of L. monocytogenes strains resistant to erythromycin or tetracycline were published in France [1Quentin C Thibout MC Horovitz J Bebear C. Multi-resistant strains of Listeria monocytogenes in septic abortion.Lancet. 1990; 336: 375Abstract PubMed Scopus (38) Google Scholar, 2Poyart-Salmeron C Carlier C Trieu-Cuot P Courtieu AL Courvalin P. Transferable plasmid-mediated antibiotic resistance in Listeria monocytogenes.Lancet. 1990; 335: 1422-1426Abstract PubMed Scopus (209) Google Scholar] and the UK [3MacGowan AP Reeves DS McLauchlin J. Antibiotic resistance of Listeria monocytogenes meningitis.Lancet. 1990; 336: 513-514Abstract Google Scholar] respectively. In one of the multiresistant strains isolated in France [2Poyart-Salmeron C Carlier C Trieu-Cuot P Courtieu AL Courvalin P. Transferable plasmid-mediated antibiotic resistance in Listeria monocytogenes.Lancet. 1990; 335: 1422-1426Abstract PubMed Scopus (209) Google Scholar] a 37-kb plasmid was found to be responsible for the resistance to the antibiotics. Genetic characterization of plasmid-encoded multiple antibiotic resistance in another L. monocytogenes strain causing endocarditis revealed that the resistance determinants were located on a 39-kb plasmid [4Hadorn K Hachler H Schaffner A Kayser FH. Genetic characterization of plasmid-encoded multiple antibiotic resistance in a strain of Listeria monocytogenes causing endocarditis.Eur J Clin Microbiol Infect Dis. 1993; 12: 928-937Crossref PubMed Scopus (63) Google Scholar]. In our laboratory, we perform antibacterial susceptibility tests by the disk diffusion method on Mueller-Hinton agar (Oxoid), on all listerial strains that we isolate from human, food and environmental sources. The zone diameters round each disk are interpreted on the basis of guidelines published by the National Committee for Clinical Laboratory Standards (NCCLS) [5National Committee for Clinical Laboratory StandardsPerformance Standards for Antimicrobial Disk Susceptibility Tests. National Committee for Clinical Laboratory Standards, Villanova, Pa1993Google Scholar]. Until 1995, all L. monocytogenes isolates were susceptible, or intermediately susceptible, to gentamicin, rifampicin, co-trimoxazole, amikacin, tobramycin, tetracycline, erythromycin, vancomycin, cefalothin, norfloxacin, ampicillin, chloramphenicol, penicillin and ampicillin/clavulanic acid. During 1995, a L. monocytogenes strain was isolated from the feces of a 9-month-old patient who suffered from diarrhea. This isolate was identified with the API Listeria kit (bioMerieux, France). It was hemolytic and agglutinated with both Listeria O polyvalent antiserum and Listeria O antiserum type 1 (Difco Laboratories. Detroit, Michigan). When checked by the disk-agar diffusion test, it was found to be resistant to gentamicin, tobramycin, tetracycline, erythomycin, chloramphenicol, amikacin and streptomycin. Minimal inhibitory concentrations (MICs) were determined as outlined in the guidelines of the NCCLS [6National Committee for Clinical Laboratory StandardsMethods for dilution antimicrobial susceptibility testing for bacteria that grow aerobically. National Committee for Clinical Laboratory Standards, Villanova, Pa1993Google Scholar], and were as follows: gentamicin 16 μg/mL, tobramycin >8 μg/mL, tetracycline >8 μg/mL, erythromycin >4 μg/mL, chloramphenicol >16 μg/mL, amikacin >32 μg/mL, streptomycin >1000 μg/mL. We isolated plasmid DNA from this strain by the alkaline lysis method [7Maniatis T Fritsch EF Sambrook J. Molecular cloning: a laboratory manual. Cold Spring Harbor Laboratory, Cold Spring Harbor, NY1982Google Scholar], and observed a single plasmid of approximately 37 kb. It was thought that L. monocytogenes was always susceptible to a number of antibiotics, and there was no need for a susceptibility test. The finding of a resistant isolate in Greece is important, paralleling discoveries of resistance in other European countries because gentamicin is given in combination with ampicillin as first-choice treatment of human infections due to L. monocytogenes and erythromycin is recommended as an alternative in pregnancy and erythromycin or tetracycline in cases of penicillin allergy. For this reason, susceptibility testing is necessary in all cases of human listeriosis.