Validation of a Gradient Diffusion Method (Etest) for Testing of Antimicrobial Susceptibility of Aerococcus urinae to Fluoroquinolones

Evaluation of gradient diffusion in vitro susceptibility testing of Aerococcus urinae

OBJECTIVE

Aerococcus urinae antimicrobial susceptibility testing can be performed via broth microdilution with Mueller-Hinton broth supplemented with lysed horse blood. We sought to compare this with the commonly used gradient diffusion method.

METHODS

We compared broth microdilution with Mueller-Hinton broth supplemented with lysed horse blood and gradient diffusion via Mueller-Hinton agar supplemented with sheep blood for 190 A. urinae isolates against 16 antimicrobials.

RESULTS

No antimicrobials demonstrated more than 90% essential and categorical agreement, and fewer than 3% demonstrated major and very major error rates. Trimethoprim-sulfamethoxazole demonstrated an 81% major error rate and ceftriaxone demonstrated a 76% very major error rate. Agar dilution with lysed horse blood was performed for trimethoprim-sulfamethoxazole against 94 isolates and showed 100% susceptibility, consistent with previous studies.

CONCLUSIONS

Given its limitations in detecting resistant strains, our findings cannot support the routine use of gradient diffusion with Mueller-Hinton agar supplemented with sheep blood for A. urinae in lieu of the Clinical and Laboratory Standards Institute method. Our results suggest that A. urinae is usually susceptible to penicillin, linezolid, tetracycline, and vancomycin. Future studies should evaluate alternative testing methods for clinical microbiology laboratories.

Acute Pyelonephritis with Bacteremia in an 89-Year-Old Woman Caused by Two Slow-Growing Bacteria: Aerococcus urinae and Actinotignum schaalii

Aerococcus urinae is an aerobic Gram-positive coccus that grows as tiny alpha-hemolytic colonies. Actinotignum schaalii is a slow-growing facultative anaerobic Gram-positive rod. These bacteria are part of the urogenital microbiota of healthy patients, but can also be involved in urinary tract infections (UTIs), particularly in elderly men and young children. Because A. urinae and A. schaalii are fastidious and are difficult to identify with phenotypic methods, they are underestimated causes of UTIs. Their growth is slow and requires a blood-enriched medium incubated under an anaerobic or 5% CO2 atmosphere for 48 h and from 24 to 48 h for A. schaalii and A. urinae, respectively. Furthermore, accurate identification is only possible using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) or molecular-based methods. In rare cases, these bacteria can be responsible for invasive infections. We describe, here, an unusual case of bacteremic UTI caused by both A. schaalii and A. urinae in an 89-year-old woman. She presented with dyspnea, and bacteriuria was noted. This challenging clinical and microbiological diagnosis was made in our laboratory by Gram staining urine with a leucocyte count >50/μL and/or a bacterial count >14/μL urinary culture on a blood agar plate. After 10 days of antimicrobial treatment consisting of 2 g amoxicillin PO t.i.d., the patient was discharged with a complete clinical and biological recovery. A. schaalii and A. urinae are probably still underestimated causes of UTIs. Microbiologists could consider the presence of these two bacteria using appropriate culture and identification methods in cases where a positive direct examination of urine reveals small Gram-positive rods or cocci, where undocumented UTIs are present in elderly patients, but also where a urinary dipstick is negative for nitrites and is associated with leukocyturia.

Susceptibility of Aerococcus urinae and Aerococcus sanguinicola to Standard Antibiotics and to Nitroxoline

Aerococcus urinae and Aerococcus sanguinicola have been increasingly recognized as causative agents of urinary tract infection (UTI) during the last decade. Nitroxoline achieves high urinary concentrations after oral administration and is recommended in uncomplicated UTI in Germany, but its activity against Aerococcus spp. is unknown. The aim of this study was to assess the in vitro susceptibility of clinical Aerococcus species isolates to standard antibiotics and to nitroxoline. Between December 2016 and June 2018, 166 A. urinae and 18 A. sanguinicola isolates were recovered from urine specimens sent to the microbiology laboratory of the University Hospital of Cologne, Germany. Susceptibility to standard antimicrobials was analyzed by disk diffusion (DD) according to EUCAST methodology, nitroxoline was tested by DD and agar dilution. Susceptibility of Aerococcus spp. to benzylpenicillin, ampicillin, meropenem, rifampicin, nitrofurantoin, and vancomycin was 100% and resistance was documented only against ciprofloxacin (20 of 184; 10.9%). MICs of nitroxoline in A. urinae isolates were low (MIC50/90 1/2 mg/L) while significantly higher MICs were observed in A. sanguinicola (MIC50/90 64/128 mg/L). If the EUCAST nitroxoline breakpoint for E. coli and uncomplicated UTI was applied (16 mg/L), 97.6% of A. urinae isolates would be interpreted as susceptible while all A. sanguinicola isolates would be considered resistant. Nitroxoline demonstrated high activity against clinical A. urinae isolates, but low activity against A. sanguinicola. Nitroxoline is an approved antimicrobial for UTI and could be an alternative oral drug to treat A. urinae urinary tract infection, yet clinical studies are needed to demonstrate this potential in vivo. IMPORTANCE A. urinae and A. sanguinicola have been increasingly recognized as causative agents in urinary tract infections. Currently, there are few data available on the activity of different antibiotics against these species and no data on nitroxoline. We demonstrate that clinical isolates in Germany are highly susceptible to ampicillin, while resistance to ciprofloxacin was common (10.9%). Additionally, we show that nitroxoline is highly active against A. urinae, but not against A. sanguinicola, which based on the presented data, should be considered intrinsically resistant. The presented data will help to improve the therapy of urinary tract infections by Aerococcus species.

Antibacterial and antibiofilm activities of silver-decorated zinc ferrite nanoparticles synthesized by a gamma irradiation-coupled sol-gel method against some pathogenic bacteria from medical operating room surfaces

This work aimed at the gamma irradiation-assisted synthesis of silver (Ag)-decorated ZnFe2O4 (ZFO) ferrite nanoparticles (NPs), which were tested for their antibacterial and antibiofilm activities against some pathogenic bacteria from medical operating room surfaces. The prepared Ag-decorated ZFO NPs were characterized via XRD, SEM, EDX, elemental mapping, and FTIR analysis. The antibacterial potential was tested as ZOI and MIC, while antibiofilm activity was estimated by the tube method. The growth curve assay, the effect of UV on the antimicrobial activity, and cell membrane leakage were evaluated, and the antibacterial reaction mechanism was investigated by SEM/EDX analysis. The XRD and FTIR results confirmed the successful preparation of Ag-decorated ZFO NPs. Antibacterial results revealed that the most potent decorated sample was Ag0.75@ZFO NPs, recording the most significant inhibition zone against Staphylococcus vitulinus (24.67 ± 0.577 mm) and low MIC (0.097 μg mL-1) against S. vitulinus. The antibiofilm activity of Ag0.75@ZFO NPs was the highest, recorded as 97.3% for S. aureus and 95.25% for Enterococcus columbae. In the case of UV exposure, bacterial growth reached the lowest grade. Finally, it was seen that the amount of cellular protein released from bacterial cells is directly proportional to the concentration of Ag0.75@ZFO NPs, which clearly explains the formation of pits in the cell membrane. The synthesized nanocomposites may find an application after mixing with operating room paints to reduce the harmful effect of pathogenic microbes and, therefore, eliminate bacterial contamination.