In vitro susceptibility of Actinobaculum schaalii to 12 antimicrobial agents and molecular analysis of fluoroquinolone resistance.
OBJECTIVE
To assess the in vitro susceptibility of Actinobaculum schaalii to 12 antimicrobial agents as well as to dissect the genetic basis of fluoroquinolone resistance.


METHODS
Forty-eight human clinical isolates of A. schaalii collected in Switzerland and France were studied. Each isolate was identified by 16S rRNA sequencing. MICs of amoxicillin, ceftriaxone, gentamicin, vancomycin, clindamycin, linezolid, ciprofloxacin, levofloxacin, moxifloxacin, co-trimoxazole, nitrofurantoin and metronidazole were determined using the Etest method. Interpretation of results was made according to EUCAST clinical breakpoints. The quinolone-resistance-determining regions (QRDRs) of gyrA and parC genes were also identified and sequence analysis was performed for all 48 strains.


RESULTS
All isolates were susceptible to amoxicillin, ceftriaxone, gentamicin, clindamycin (except three), vancomycin, linezolid and nitrofurantoin, whereas 100% and 85% were resistant to ciprofloxacin/metronidazole and co-trimoxazole, respectively. Greater than or equal to 90% of isolates were susceptible to the other tested fluoroquinolones, and only one strain was highly resistant to levofloxacin (MIC ≥32 mg/L) and moxifloxacin (MIC 8 mg/L). All isolates that were susceptible or low-level resistant to levofloxacin/moxifloxacin (n = 47) showed identical GyrA and ParC amino acid QRDR sequences. In contrast, the isolate exhibiting high-level resistance to levofloxacin and moxifloxacin possessed a unique mutation in GyrA, Ala83Val (Escherichia coli numbering), whereas no mutation was present in ParC.


CONCLUSIONS
When an infection caused by A. schaalii is suspected, there is a risk of clinical failure by treating with ciprofloxacin or co-trimoxazole, and β-lactams should be preferred. In addition, acquired resistance to fluoroquinolones more active against Gram-positive bacteria is possible.
Cattoir
Vincent
V
Service de Microbiologie, CHU Côte de Nacre, Caen, France. cattoir-v@chu-caen.fr
Varca
Alexandre
A
Greub
Gilbert
G
Prod'hom
Guy
G
Legrand
Patrick
P
Lienhard
Reto
R
D000191:Actinomycetaceae / Q000145:classification / Q000187:drug effects* / Q000201:enzymology / Q000235:genetics; D000193:Actinomycetales Infections / Q000382:microbiology; D000595:Amino Acid Sequence; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D002939:Ciprofloxacin / Q000494:pharmacology; D027081:DNA Gyrase / Q000235:genetics*; D027101:DNA Topoisomerase IV / Q000235:genetics*; D024881:Drug Resistance, Bacterial / Q000235:genetics*; D024841:Fluoroquinolones / Q000494:pharmacology*; D005602:France; D006801:Humans; D008826:Microbial Sensitivity Tests; D008969:Molecular Sequence Data; D009154:Mutation; D017422:Sequence Analysis, DNA; D013557:Switzerland; D015662:Trimethoprim, Sulfamethoxazole Drug Combination / Q000494:pharmacology; D014552:Urinary Tract Infections / Q000382:microbiology
D000900:Anti-Bacterial Agents; D024841:Fluoroquinolones; D002939:Ciprofloxacin; D015662:Trimethoprim, Sulfamethoxazole Drug Combination; D027101:DNA Topoisomerase IV; D027081:DNA Gyrase