David C. Bean

Antimicrobial resistance in community and nosocomial Escherichia coli urinary tract isolates, London 2005-2006.

BackgroundEscherichia coli is the commonest cause of community and nosocomial urinary tract infection (UTI). Antibiotic treatment is usually empirical relying on susceptibility data from local surveillance studies. We therefore set out to determine levels of resistance to 8 commonly used antimicrobial agents amongst all urinary isolates obtained over a 12 month period.MethodsAntimicrobial susceptibility to ampicillin, amoxicillin/clavulanate, cefalexin, ciprofloxacin, gentamicin, nitrofurantoin, trimethoprim and cefpodoxime was determined for 11,865 E. coli urinary isolates obtained from community and hospitalised patients in East London.ResultsNitrofurantoin was the most active agent (94% susceptible), followed by gentamicin and cefpodoxime. High rates of resistance to ampicillin (55%) and trimethoprim (40%), often in combination were observed in both sets of isolates. Although isolates exhibiting resistance to multiple drug classes were rare, resistance to cefpodoxime, indicative of Extended spectrum β-lactamase production, was observed in 5.7% of community and 21.6% of nosocomial isolates.ConclusionWith the exception of nitrofurantoin, resistance to agents commonly used as empirical oral treatments for UTI was extremely high. Levels of resistance to trimethoprim and ampicillin render them unsuitable for empirical use. Continued surveillance and investigation of other oral agents for treatment of UTI in the community is required.

Bloodstream infection due to Acinetobacter spp: epidemiology, risk factors and impact of multi-drug resistance

Acinetobacter spp. are increasingly reported as important causes of human infection. Many isolates exhibit multi-drug resistance, raising concerns over our ability to treat serious infections with these organisms. The impact of infection on clinical outcome as well as the importance of multi-drug resistance is poorly defined. A descriptive retrospective observational study was undertaken of all episodes of Acinetobacter bacteremia occurring in a UK tertiary care centre from 1998–2006. Demographics of infected patients, characteristics and antimicrobial susceptibility of infecting strains were recorded and the impact of antimicrobial therapy on all causes of 30-day mortality assessed. Three hundred ninety-nine episodes of Acinetobacter bacteremia were identified, with A. baumannii being the most frequently isolated species. Most episodes occurred in critical care and were associated with multidrug resistance, with carbapenem resistance rising from 0% in 1998 to 55% in 2006. Although bacteremia due to carbapenem-resistant Acinetobacter and a requirement for critical care were associated with a higher mortality, mortality was not reduced by the administration of appropriate empirical antimicrobial therapy. A prospective study is required to identify both the most effective intervention and those most likely to benefit from treatment.

In-vitro activity of polymyxin B in combination with imipenem, rifampicin and azithromycin versus multidrug resistant strains of Acinetobacter baumannii producing OXA-23 carbapenemases

BackgroundAcinetobacter baumannii has emerged as a major nosocomial pathogen worldwide. Many of the circulating strains exhibit multi-drug resistance remaining consistently susceptible only to polymyxins. In-vitro studies have reported that polymyxins combined with carbapenems, rifampicin or azithromycin are synergistic against these strains despite in-vitro resistance to these agents alone. The use of antimicrobial combinations have therefore been advocated for the treatment of severe A. baumannii infection in man. In order to determine whether such combinations are synergistic against the prevalent clones of multi-drug resistant A. baumannii causing infection in the UK, we performed synergy testing against representative isolates using two rapid Etest methods.MethodsThe activity of polymyxin in combination with imipenem, azithromycin or rifampicin was assessed against five strains of multi-drug resistant A. baumannii encoding OXA-23 carbapenemases. Synergy studies were performed by Etest-agar dilution and a combined Etest strip method. Synergy was defined as a FICI of ≤ 0.5.ResultsAll strains were resistant to β-lactams, carbapenems, quinolones and aminoglycosides but susceptible to polymyxins. Marked synergy was not seen with polymyxin in combination with imipenem, rifampicin or azithromycin against any of the strains. Borderline synergy (FICI = 0.5) was seen against one strain belonging to OXA-23 clonal group 2, using the Etest-agar dilution method only.ConclusionIn-vitro synergy with polymxyin in combination with imipenem, rifampicin or azithromycin is highly strain and method dependent. As reliable synergy could not be demonstrated against the prevalent UK multi-drug resistant strains, use of such combinations should not be used for empirical treatment of these infections in the UK. The optimal treatment for serious multi-drug A. baumannii infection and the role of combination therapy should be addressed in a prospective clinical trial.

Paradoxical effect of 1-(1-naphthylmethyl)-piperazine on resistance to tetracyclines in multidrug-resistant Acinetobacter baumannii

OBJECTIVES The efflux inhibitor 1-(1-naphthylmethyl)-piperazine (NMP) has been demonstrated to reverse multidrug resistance in Acinetobacter baumannii. We investigated the interaction of NMP with tigecycline and three other tetracyclines on clinical isolates of A. baumannii. METHODS One hundred and four clinical isolates of Acinetobacter were tested for susceptibility to tigecycline, minocycline, doxycycline and tetracycline by disc diffusion, and tigecycline MICs were determined by Etest, both in the presence and absence of NMP. Tigecycline MICs and zones of inhibition were interpreted using the BSAC guidelines. An OXA carbapenemase multiplex PCR was also performed on each isolate. RESULTS Mean zones of inhibition for tetracycline, doxycycline and minocycline increased by 11.3%, 22.9% and 11.2%, respectively, in the presence of NMP. In contrast, tigecycline susceptibility was decreased in the presence of NMP, with mean zones of inhibition decreasing by 8.4%. Based on PCR results, all but six isolates belonged to the OXA-23 clone 1. CONCLUSIONS Susceptibility to tigecycline of the A. baumannii OXA-23 clone 1 prevalent in the UK is reduced (approximately 2-fold) by the presence of the efflux inhibitor NMP. NMP does not have the same effect on susceptibility to other tetracyclines.

In Vitro Activities of Polymyxin B, Imipenem, and Rifampin against Multidrug-Resistant Acinetobacter baumannii

In a previous issue of this journal, Yoon et al. reported the activities of polymyxin B, imipenem, and rifampin in double and triple combinations against multidrug-resistant strains of Acinetobacter baumannii (7). These organisms have emerged as important nosocomial pathogens responsible for outbreaks of pneumonia, bacteremia, and sepsis among critically ill patients throughout the world (2, 3). As treatment options are limited, studies demonstrating enhanced activity with combinations of one or more agents are welcomed as they may be useful in selecting and guiding therapy. Yoon et al. have used standard checkerboard microtiter plate and time-kill assays against eight isolates of A. baumannii and conclude that bactericidal synergy can be obtained using polymyxin B in combination with imipenem or rifampin and using all three agents combined. In drawing their conclusions, Yoon et al. have defined synergy in checkerboard assays as a fractional inhibitory concentration index (FICI) of <1.0. We are concerned that this interpretation of the FICI overemphasizes the significance of their findings. It is widely accepted that variability in MIC determinations means the true value may lie within a three-dilution range (6). When testing two antibiotics, this effect is cumulative, and the errors are subsequently incorporated in the FICI score. In view of this, the editorial policies of many journals, including Antimicrobial Agents and Chemotherapy (1) and the Journal of Antimicrobial Chemotherapy (5), require FICI data of <0.5 to be defined as synergy. We feel the inclusion of a third antibiotic is not sufficient reason to merit the use of different criteria for interpreting FICI data. In fact, these scores will carry even greater inbuilt error and should therefore be interpreted at least as conservatively as FICI data derived from testing with two antibiotics. If the results of Yoon et al. are reinterpreted using these criteria, then only three of their double combinations and two of their triple combinations are in fact synergistic, with the remainder having only additive, if any, effect. Some authors have suggested more stringent criteria, with a FICI of ≤0.25 obtained in at least five replicate experiments (6). If these criteria are used, then none of the combinations tested by Yoon et al. are synergistic. Combinations of imipenem, rifampin, and colistin have been found to be effective in animal models (4), and these combinations may yet prove to be the most effective means of treating multidrug-resistant Acinetobacter infections. However, until there is a well-designed clinical trial of the safety and efficacy of these regimens, clinicians should remain cautious in extrapolating the existing in vitro and animal data to humans.

Listeriosis Due to Infection with a Catalase-Negative Strain of Listeria monocytogenes

ABSTRACT A strain of Listeria monocytogenes recovered from blood and cerebrospinal fluid had no detectable catalase activity, a characteristic used for primary identification. The sporadic occurrence of pathogenic catalase-negative strains highlights the need for a reconsideration of diagnostic criteria and questions the role of catalase in the pathogenesis of listeria infection.

CHROMagar COL-APSE: a selective bacterial culture medium for the isolation and differentiation of colistin-resistant Gram-negative pathogens.

Purpose. A selective chromogenic culture medium for the laboratory isolation and differentiation of colistin resistant Acinetobacter, Pseudomonas, Stenotrophomonas and Enterobacteriaceae spp. (CHROMagar COL‐APSE) was developed, evaluated and compared to an existing selective bacterial culture medium (SuperPolymyxin). Methodology. The medium was challenged with 84 isolates, including polymyxin B (POL B)‐susceptible and ‐resistant type strains and colistin (COL)‐resistant organisms recovered from human and animal samples. Susceptibility to COL and POL B was determined by agar dilution and broth microtitre dilution. The lower limit for the detection of COL‐resistant organisms was also calculated for both CHROMagar COL‐APSE and SuperPolymyxin media. The ability to isolate and correctly differentiate COL‐resistant organisms within mixed cultures was also assessed and compared using both media. Results. Using CHROMagar COL‐APSE, Gram‐negative pathogens (n=71) with intrinsic (n=8) or acquired COL (n=63) resistance were recovered with 100% specificity down to the lower limit of detection of 101 colony‐forming units (c.f.u.). The growth on SuperPolymyxin was similar, but notably weaker for COL‐resistant non‐fermentative bacteria (Acinetobacter, Pseudomonas and Stenotrophomonas). CHROMagar COL‐APSE was also more sensitive in supporting the growth of Enterobacteriaceae with COL resistance associated with the carriage of mcr‐1. Conclusion. CHROMagar COL‐APSE is a sensitive and specific medium for the growth of COL‐resistant bacterial pathogens. Due to the low limit of detection (101 c.f.u.), it may be useful as a primary isolation medium in the surveillance and recovery of COL‐resistant bacteria from complex human, veterinary and environmental samples, especially those with plasmid‐mediated MCR‐1 or novel mechanisms of polymyxin resistance.

Chocolate and Confectionary

Confectionary products are often considered relatively inert from a microbiological perspective. Nevertheless, several Salmonella outbreaks have been attributed to confectionary, particularly chocolate products. The cause of these outbreaks was generally traced back to lapses in GMP, particularly cross contact issues and water ingress. Managing Salmonella in chocolate manufacture begins with a validated cocoa bean-roasting process. However, the potential for pathogen recontamination exists with the addition of ingredients and inclusions post process. This risk can be managed by a stringent supplier assurance program including prerelease microbiological testing of these materials. In addition to assured ingredients, the manufacturing environment must include a strict containment policy for raw and finished goods, control of water use including the prevention of water leaks, and ongoing microbial surveillance. Manufacturing equipment needs to be hygienically designed and amenable to sanitation processes, should a contamination event occur. Lastly, an effective microbiological verification program is essential to ensure all described processes are in control.

Antimicrobial sensitivity trends and virulence genes in Shigella spp. from the Oceania region

Shigella is a common cause of diarrhoea in Papua New Guinea (PNG) and other Oceania countries. However, little is known about the strains causing infection. Archived Shigella isolates (n = 72) were obtained from research laboratories in PNG and reference laboratories in Australia. Shigella virulence genes were detected by PCR, and antimicrobial susceptibility was determined by disk diffusion. The ipaH virulence gene was present in all 72 isolates. The prevalence of other virulence genes was variable, with ial, invE, ipaBCD, sen/ospD3 and virF present in 60% of isolates and set1A and set1B genes present in 42% of isolates. Most S. flexneri isolates contained genes encoding enterotoxin 1 and/or enterotoxin 2. Resistance to antibiotics was common, with 51/72 isolates resistant to 2-4 antimicrobials. A greater proportion of bacteria isolated since 2010 (relative to pre-2010 isolates) were resistant to commonly used antibiotics such as ampicillin, chloramphenicol, tetracycline, and trimethoprim-sulfamethoxazole; suggesting that antimicrobial resistance (AMR) in Shigella is increasing over time in the Oceania region. There is a need for improved knowledge regarding Shigella circulation in the Oceania region and further monitoring of AMR patterns.

Pentamidine: a drug to consider re-purposing in the targeted treatment of multi-drug resistant bacterial infections?

The recent emergence of plasmid-mediated colistin resistance (1) highlights a new problem in the development of bacterial antimicrobial resistance. Colistin, while an old antibiotic, is increasingly turned to as a “drug of last resort” for many recalcitrant Gram-negative infections. With the efficacy of this drug now compromised, the spread of pan-resistant organisms is a very real threat. To meet this challenge clinicians are seeking new treatment options, however with a drug development pipeline that is virtually dry, an urgent rethinking of how we employ our existing arsenal of antibiotics is required to deal with this problem. Un-orthodox combinations of existing antimicrobial drugs are increasingly being used, often with little rationale on how such combinations are selected beyond the thinking that ‘more is likely to be better’ (2). There is a clear need for a precision medicine approach that combines robust laboratory testing methods with effective dosing strategies for individual patients.