Amanda J. Hall

Minimum Inhibitory Concentrations of Standard and Novel Antimicrobials for Isolates from Bacterial Keratitis

PURPOSE To determine the minimum inhibitory concentrations (MICs) of 12 antimicrobials in current ophthalmic use and 4 potentially new alternatives against isolates from bacterial keratitis. METHODS Bacteria were collected from cases of bacterial keratitis in six centers in the United Kingdom between 2003 and 2006. MICs were measured by using susceptibility strips containing a concentration gradient of the antimicrobials penicillin, cefuroxime, ceftazidime, chloramphenicol, gentamicin, amikacin, vancomycin, teicoplanin, ciprofloxacin, ofloxacin, levofloxacin, moxifloxacin, meropenem, linezolid, tigecycline, and daptomycin. RESULTS Isolates (n = 772) were collected including coagulase negative Staphylococcus (CNS) (30%), Pseudomonas aeruginosa (23%), Staphylococcus aureus (14%), Enterobacteriaceae (14%), and streptococci (13%). Meropenem had low MICs for most isolates. All isolates except P. aeruginosa were susceptible to tigecycline. Linezolid was active against the majority of Gram-positive pathogens. Ten percent of S. aureus and 20% of CNS isolates were methicillin resistant. When systemic breakpoints were used, 84% of S. aureus isolates were susceptible to ciprofloxacin and 98% to moxifloxacin. Of the P. aeruginosa isolates, 99% were susceptible to ceftazidime, 96% to gentamicin, 99% to ciprofloxacin and 100% to moxifloxacin. More than 97% of Enterobacteriaceae isolates were susceptible to ceftazidime, gentamicin, ciprofloxacin, and moxifloxacin. CONCLUSIONS Based on systemic breakpoint data, resistance to commonly used antimicrobials was apparent. Meropenem is a potentially effective agent for ophthalmic use, with low MICs throughout all the bacterial subgroups. Tigecycline and linezolid showed good activity against particular groups and may be useful for treating bacterial keratitis resistant to current antimicrobials. Of the fluoroquinolones, moxifloxacin showed the lowest MICs and resistance for both Gram-positive and -negative bacteria.

Developing an international Pseudomonas aeruginosa reference panel

Pseudomonas aeruginosa is a major opportunistic pathogen in cystic fibrosis (CF) patients and causes a wide range of infections among other susceptible populations. Its inherent resistance to many antimicrobials also makes it difficult to treat infections with this pathogen. Recent evidence has highlighted the diversity of this species, yet despite this, the majority of studies on virulence and pathogenesis focus on a small number of strains. There is a pressing need for a P. aeruginosa reference panel to harmonize and coordinate the collective efforts of the P. aeruginosa research community. We have collated a panel of 43 P. aeruginosa strains that reflects the organisms diversity. In addition to the commonly studied clones, this panel includes transmissible strains, sequential CF isolates, strains with specific virulence characteristics, and strains that represent serotype, genotype or geographic diversity. This focussed panel of P. aeruginosa isolates will help accelerate and consolidate the discovery of virulence determinants, improve our understanding of the pathogenesis of infections caused by this pathogen, and provide the community with a valuable resource for the testing of novel therapeutic agents.

An In Vitro Investigation of Synergy or Antagonism between Antimicrobial Combinations against Isolates from Bacterial Keratitis

PURPOSE To investigate antimicrobial combinations for synergy or antagonism against isolates of Staphylococcus aureus and Pseudomonas aeruginosa. METHODS Isolates were collected from cases of microbial keratitis from six centers in the United Kingdom. Minimum inhibitory concentrations (MICs) were determined by using E-test strips for 16 antimicrobials, including both current and potentially available agents. E-test strips were used to test selected antimicrobials in combination against a representative set of 10 S. aureus and 10 P. aeruginosa isolates. E-tests of the two antimicrobials were placed on sensitivity agar at right angles intersecting at their respective MICs. Antimicrobial combinations were classified as synergistic, additive, indifferent, or antagonistic, according to their fractional inhibitory concentration (FIC). RESULTS The combinations meropenem and ciprofloxacin, meropenem and teicoplanin, moxifloxacin and teicoplanin, and ciprofloxacin and teicoplanin, gave the lowest mean FICs for S. aureus, with synergy or additivity being seen in 60% to 80% of isolates. The meropenem/ciprofloxacin combination gave the lowest mean FIC for P. aeruginosa isolates, with 90% showing an additive or synergistic effect. The other combinations elicited a predominantly indifferent response. No consistent antagonistic effect was observed with the combinations used. CONCLUSIONS The combination of meropenem and ciprofloxacin was predominantly additive or synergistic for both S. aureus and P. aeruginosa. Teicoplanin combined with meropenem, ciprofloxacin, or moxifloxacin was also predominantly additive or synergistic against S. aureus.

Differential infection properties of three inducible prophages from an epidemic strain of Pseudomonas aeruginosa

BackgroundPseudomonas aeruginosa is the most common bacterial pathogen infecting the lungs of patients with cystic fibrosis (CF). The Liverpool Epidemic Strain (LES) is transmissible, capable of superseding other P. aeruginosa populations and is associated with increased morbidity. Previously, multiple inducible prophages have been found to coexist in the LES chromosome and to constitute a major component of the accessory genome not found in other sequenced P. aerugionosa strains. LES phages confer a competitive advantage in a rat model of chronic lung infection and may, therefore underpin LES prevalence. Here the infective properties of three LES phages were characterised.ResultsThis study focuses on three of the five active prophages (LESφ2, LESφ3 and LESφ4) that are members of the Siphoviridae. All were induced from LESB58 by norfloxacin. Lytic production of LESφ2 was considerably higher than that of LESφ3 and LESφ4. Each phage was capable of both lytic and lysogenic infection of the susceptible P. aeruginosa host, PAO1, producing phage-specific plaque morphologies. In the PAO1 host background, the LESφ2 prophage conferred immunity against LESφ3 infection and reduced susceptibility to LESφ4 infection. Each prophage was less stable in the PAO1 chromosome with substantially higher rates of spontaneous phage production than when residing in the native LESB58 host. We show that LES phages are capable of horizontal gene transfer by infecting P. aeruginosa strains from different sources and that type IV pili are required for infection by all three phages.ConclusionsMultiple inducible prophages with diverse infection properties have been maintained in the LES genome. Our data suggest that LESφ2 is more sensitive to induction into the lytic cycle or has a more efficient replicative cycle than the other LES phages.

Calprotectin and Lactoferrin Faecal Levels in Patients with Clostridium difficile Infection (CDI): A Prospective Cohort Study

Measurement of both calprotectin and lactoferrin in faeces has successfully been used to discriminate between functional and inflammatory bowel conditions, but evidence is limited for Clostridium difficile infection (CDI). We prospectively recruited a cohort of 164 CDI cases and 52 controls with antibiotic-associated diarrhoea (AAD). Information on disease severity, duration of symptoms, 30-day mortality and 90-day recurrence as markers of complicated CDI were recorded. Specimens were subject to microbiological culture and PCR-ribotyping. Levels of faecal calprotectin (FC) and lactoferrin (FL) were measured by ELISA. Statistical analysis was conducted using percentile categorisation. ROC curve analysis was employed to determine optimal cut-off values. Both markers were highly correlated with each other (r2 = 0.74) and elevated in cases compared to controls (p<0.0001; ROC>0.85), although we observed a large amount of variability across both groups. The optimal case-control cut-off point was 148 mg/kg for FC and 8.1 ng/µl for FL. Median values for FL in CDI cases were significantly greater in patients suffering from severe disease compared to non-severe disease (104.6 vs. 40.1 ng/µl, p = 0.02), but were not significant for FC (969.3 vs. 512.7 mg/kg, p = 0.09). Neither marker was associated with 90-day recurrence, prolonged CDI symptoms, positive culture results and colonisation by ribotype 027. Both FC and FL distinguished between CDI cases and AAD controls. Although FL was associated with disease severity in CDI patients, this showed high inter-individual variability and was an isolated finding. Thus, FC and FL are unlikely to be useful as biomarkers of complicated CDI disease.

Molecular epidemiological analysis suggests cross-infection with Pseudomonas aeruginosa is rare in non-cystic fibrosis bronchiectasis

To the Editor: In both cystic fibrosis (CF) and non-CF bronchiectasis (NCFBr) chronic Pseudomonas aeruginosa infection is adversely prognostic [1, 2]. In CF, epidemic infections with specific clones of P. aeruginosa are associated with further adverse outcomes [3, 4]. This cross-infection risk has led to segregation of patients [5]. There are few data on P. aeruginosa cross-infection in NCFBr. As a result, segregation in NCFBr has not been addressed in guidelines [6]. Our aim was to undertake a cross-infection study in NCFBr. This was undertaken in an adult bronchiectasis service in the north-east of England (UK) that is separated from the regional CF unit (sited 2 miles (3 km) away). The service was initiated in 2007 with a weekly specialist clinic without a Pseudomonas -specific clinic. When NCFBr patients are hospitalised, there is a preference for cubicle-based (single-patient room) management, but when cubicles are unavailable, patients are managed in six-bed bays. All patients had computed tomographic confirmation and had predominantly idiopathic or post-infectious bronchiectasis with CF excluded following current guidelines [6]. The study had ethical permission and Caldicott approval (Newcastle and North Tyneside National Research Ethics Service Committee). 56 isolates were selected for analysis. Six were chosen from CF patients as laboratory controls. 50 were NCFBr isolates collected between 2008 and 2011 from …

Intraclonal genetic diversity amongst cystic fibrosis and keratitis isolates of Pseudomonas aeruginosa

Given the emergence of transmissible strains of Pseudomonas aeruginosa, such as the Liverpool epidemic strain (LES), in cystic fibrosis (CF) centres, it is important to carry out regular surveillance of isolates. In a survey of 22 P. aeruginosa isolates, each from a different CF patient identified as negative for LES in a paediatric centre in Liverpool, six (23 %) were identified as being the same clone type (clone D) using array-tube genotyping. Using a series of alternative genotyping approaches [PFGE, random amplification of polymorphic DNA (RAPD), variable number of tandem repeats (VNTR) and multilocus sequence typing (MLST)], the six CF clone D isolates and eight previously identified clone D isolates associated with infections leading to keratitis were compared. All but two of the clone D isolates (both keratitis-associated) were assigned by MLST to sequence type 235 and were highly similar using VNTR analysis. However, there was considerable variation found among the isolates when using PFGE or RAPD, highlighting the limitations of these methods. The discordance with respect to two of the isolates identified by array-tube genotyping as clone D, when using all the other typing methods, emphasizes the need to use more than one method for reliable identification of strains.

Divergence of a strain of Pseudomonas aeruginosa during an outbreak of ovine mastitis.

Bacterial infections causing mastitis in sheep can result in severe economic losses for farmers. A large survey of milk samples from ewes with mastitis in Sardinia, Italy, indicated an increasing prevalence of Pseudomonas aeruginosa infections. It has been shown previously that during chronic, biofilm-associated infections P. aeruginosa populations diversify. We report the phenotypic and genomic characterisation of two clonal P. aeruginosa isolates (PSE305 and PSE306) from a mastitis infection outbreak, representing distinct colony morphology variants. In addition to pigment production, PSE305 and PSE306 differed in phenotypic characteristics including biofilm formation, utilisation of various carbon and nitrogen sources, twitching motility. We found higher levels of expression of genes associated with biofilm formation (pelB) and twitching motility (flgD) in PSE305, compared to the biofilm and twitching-defective PSE306. Comparative genomics analysis revealed single nucleotide polymorphisms (SNPs) and minor insertion/deletion variations between PSE305 and PSE306, including a SNP mutation in the pilP gene of PSE306. By introducing a wild-type pilP gene we were able to partially complement the defective twitching motility of PSE306. There were also three larger regions of difference between the two genomes, indicating genomic instability. Hence, we have demonstrated that P. aeruginosa population divergence can occur during an outbreak of mastitis, leading to significant variations in phenotype and genotype, and resembling the behaviour of P. aeruginosa during chronic biofilm-associated infections.

Turnover of strains and intraclonal variation amongst Pseudomonas aeruginosa isolates from paediatric CF patients

Transmissible strains of Pseudomonas aeruginosa in cystic fibrosis (CF) have been well documented. Our longitudinal survey of P. aeruginosa isolates from 45 paediatric CF patients indicated strain persistence and intraclonal diversity, but not cross infection. This study demonstrates the need for regular P. aeruginosa strain surveillance using genotyping.

S126 Molecular epidemiological analysis suggests cross infection with pseudomonas aeruginosa is rare in non-cystic fibrosis bronchiectasis

Background Non Cystic Fibrosis Bronchiectasis (NCFBr) is a cause of significant morbidity and mortality. Pseudomonas aeruginosa, a key pathogen in NCFBr, is associated with premature mortality. Globally, common clones of P. aeruginosa have been recognised from clinical and environmental sources and in Cystic Fibrosis (CF) cross infection is known to occur. There are no robust data on cross infection in NCFBr. This evidence gap impacts on managing patients but was omitted from the BTS 2010 guidelines due to the paucity of data. Aims To seek evidence of cross infection amongst NCFBr patients. Methods Single centre cross sectional study: We studied 50 P. aeruginosa isolates from 40 NCFBr patients using two genotyping techniques (both blinded); an Array Tube (AT) method and Variable Number Tandem Repeat (VNTR) analysis. We included known CF clonal strains as internal controls. We then compared the data using genotype databases. Results This is the largest cross infection study to our knowledge. We demonstrated that shared P. aeruginosa NCFBr genotypes were infrequent. Twelve patient isolates did not match any other isolate within the NCFBr collection or the databases. The most common clone, clone C (10%), is also known to be abundant in the environment. In ten patients where longitudinal isolates were examined, paired isolates gave matching genotyping data suggesting persistent infection. There was incomplete concordance between the Array-Tube and VNTR methods (88% agreement). Conclusion There were no dominant Pseudomonas aeruginosa clones in NCFBr suggesting that the most prevalent mode of infection is sporadic and cross infection is rare. This may reflect the local infection control measures however. Multicentre studies are suggested to further assess the risks.