Characterization of a chromosomal gene encoding type B beta-lactamase in phage group II isolates of Staphylococcus aureus

β-Lactam Inoculum Effect in Methicillin-Susceptible Staphylococcus aureus Infective Endocarditis

Importance

Infective endocarditis (IE) caused by Staphylococcus aureus is associated with high mortality, approximately 20% to 30%, mostly in the first month, with no improvement in recent decades. Current opinion is that antistaphylococcal penicillin and cefazolin are equally effective in treating methicillin-susceptible S aureus (MSSA) IE, and both are recommended as possible first-line treatments. Most MSSA strains carry the β-lactamase blaZ gene, and some blaZ-positive strains exhibit an inoculum effect, meaning increased minimum inhibitory concentrations at high inoculum. This reduced susceptibility to an antibiotic at high bacterial inoculum may be particularly relevant in IE, where vegetations have very high bacterial densities.

Objective

To evaluate the association between phenotypic characteristics of S aureus isolates, β-lactam used, and outcome in patients with MSSA IE.

Design, Settings, and Participants

This retrospective case series included MSSA cases treated at 3 French university hospitals between February 2016 and February 2022. The study included patients who had clinical isolates available and had definite or possible S aureus IE that involved native or prosthetic valves. Data were analyzed from July 2023 to June 2024.

Main Outcomes and Measures

MSSA isolates were tested for the presence of blaZ and for inoculum effects to cefazolin and oxacillin. The association between first-month mortality and the β-lactam used, the presence of blaZ, and the presence of an inoculum effect to the treatment received was evaluated.

Results

This study included 216 patients with MSSA IE (median [IQR] age, 65 [49-73] years; 152 [70.4%] male) who were treated with antistaphylococcal penicillin (139 [64.4%]) or cefazolin (77 [35.6%]). One-month mortality of left-sided IE was 44 of 180 patients (24.4%), with no overall difference between patients treated with antistaphylococcal penicillin or cefazolin. However, 1-month mortality was higher in patients infected with blaZ-positive strains than with blaZ-negative strains (38 of 129 [29.5%] vs 6 of 51 [11.8%]; P = .01), and with strains with an inoculum effect to the β-lactam received than with strains without an inoculum effect (25 of 62 [40.3%] vs 13 of 67 [19.4%]; P = .005). On multivariable analysis, the presence of an inoculum effect was independently associated with first-month mortality (HR, 2.84; 95% CI, 1.28-6.30; P = .01).

Conclusions and Relevance

In this case series of MSSA IE, the presence of an inoculum effect to the β-lactam received was a risk factor for death in the first month. Phenotyping MSSA isolates for inoculum effect may guide β-lactam choice and improve outcomes.

Epidemiology and impact of methicillin-sensitive Staphylococcus aureus with β-lactam antibiotic inoculum effects in adults with cystic fibrosis

Staphylococcus aureus is the most prevalent cystic fibrosis (CF) pathogen. Several phenotypes are associated with worsened CF clinical outcomes including methicillin-resistance and small-colony-variants. The inoculum effect (IE) is characterized by reduced β-lactam susceptibility when assessed at high inoculum. The IE associates with worse outcomes in bacteremia and other high-density infections, and may therefore be relevant to CF. The prevalence of IE amongst a CF cohort (age ≥18 years), followed from 2013 to 2016, was investigated. Yearly methicillin-sensitive S. aureus (MSSA) isolates were screened at standard (5 × 105 CFU/mL) and high (5 × 107 CFU/mL) inoculum against narrow-spectrum anti-Staphylococcal β-lactams and those with anti-pseudomonal activity common to CF. A ≥ 4-fold increase in minimum inhibitory concentration between standard and high inoculum defined IE. Isolates underwent blaZ sequencing and genotyping and were compared against published genomes. Fifty-six percent (99/177) of individuals had MSSA infection. MSSA was observed at ≥105 CFU/mL in 44.8% of entry sputum samples. The prevalence of the IE was 25.0%-cefazolin; 13.5%-cloxacillin; 0%-meropenem; 1.0%-cefepime; 5.2%-ceftazidime; and 34.4%-piperacillin-tazobactam amongst baseline MSSA isolates assessed. blaZ A associated with cefazolin IE (P = 0.0011), whereas blaZ C associated with piperacillin-tazobactam IE (P < 0.0001). Baseline demographics did not reveal specific risk factors for IE-associated infections, nor were long-term outcomes different. Herein, we observed the IE in CF-derived MSSA disproportionally for cefazolin and piperacillin-tazobactam and this phenotype strongly associated with underlying blaZ genotype. The confirmation of CF being a high density infection, and the identification of high prevalence of MSSA with IE in CF supports the need for prospective pulmonary exacerbation treatment studies to understand the impact of this phenotype.

Disc Test for Detecting Staphylococcus aureus Strains Producing Type A and Type C β-Lactamases

Staphylococcus aureus can produce β-lactamases capable of hydrolyzing penicillins and first-generation cephalosporins. The propensity of type A and type C β-lactamase-producing S. aureus (TAPSA and TCPSA) to hydrolyze cefazolin at a high inoculum is termed the cefazolin inoculum effect (CIE). Strains with a CIE have a theoretical risk of causing treatment failure and are unable to be detected routinely by most laboratories. We developed a high-performing yet straightforward β-lactamase disc test that identifies and differentiates both TAPSA and TCPSA and is suitable for routine diagnostic laboratory workflows. Clinical isolates of S. aureus resistant to penicillin were identified, and their blaZ genes were sequenced. MICs were determined at low and high inocula (5 × 105 CFU/mL and 5 × 107 CFU/mL), and isolates demonstrating a CIE were characterized. A semimechanistic model was established to describe differential hydrolysis patterns, and candidate models were iteratively assessed using area-under-the-curve analysis from competitor receiver operating characteristic (ROC) curves. Biomarker thresholds were derived from Youdon index-derived optimal cutoff values. Genetic analysis of 99 isolates identified 26 TAPSA isolates and 45 TCPSA isolates. The model best differentiating TAPSA from non-TAPSA utilized cefazolin-to-cephalothin ratio analysis (sensitivity, 96.2%; specificity, 98.6%). The model best differentiating TCPSA from non-TCPSA incorporated cefazolin, cephalothin, and oxacillin (sensitivity, 88.6%; specificity, 96.6%). TAPSA and TCPSA can be differentiated using three antibiotic discs on a single agar plate. The test has potential value in typing the β-lactamase type from isolates from patients that are candidates for or have failed cefazolin therapy. IMPORTANCE The key significance of this article is that it details a straightforward method of performing a disc test that can differentiate Staphylococcus aureus isolates that are likely to be associated with a cefazolin inoculum effect and theoretical risk of cefazolin treatment failure from isolates that are less likely to be associated with a cefazolin inoculum effect.

Prevalence of a cefazolin inoculum effect associated with blaZ gene types, and clinical outcomes among methicillin-susceptible Staphylococcus aureus blood isolates of patients with infective endocarditis

OBJECTIVES

A proportion of blaZ gene-positive methicillin-susceptible Staphylococcus aureus (MSSA) strains exhibits the cefazolin inoculum effect (CInE). Its clinical impact remains uncertain but could compromise the use of cefazolin in high-burden infections. To date, no study has been conducted in France or in Europe. We aimed to assess the prevalence of CInE and its association with blaZ beta-lactamase and S. aureus protein A (spa) types, and to assess the clinical outcomes in cefazolin-treated patients for infective endocarditis whose strain exhibited a CInE.

METHODS

This was a French single-center retrospective study of 51 MSSA strains from patients of the Nantes endocarditis prospective cohort, conducted between 2013 and 2018.

RESULTS

Cefazolin MIC₅₀ at high inoculum was 2 mg/L (IQR 1-2). CInE was found in 17.6 % of tested strains. Among blaZ-positive strains (n = 44), type A beta-lactamase was predominant (n = 25, 57 %). Thirty-seven S. aureus protein A (spa) types were found. No statistical association was shown between blaZ or spa types and CInE. CInE was neither associated with a higher rate of persistent bacteremia (25 % vs 56.3 %, p = 0.58) nor with clinical failure in patients treated with cefazolin, in comparison to patients with no CInE strain (25 % vs 56.3 %, p = 0.58).

CONCLUSION

The cefazolin inoculum effect was found in a substantial number of Staphylococcus aureus strains; however, minimum inhibitory concentrations remained globally low. CInE was not associated with a higher proportion of clinical failure on treatment.

Staphylococcus aureus in Non-Cystic Fibrosis Bronchiectasis: Prevalence and Genomic Basis of High Inoculum Beta-Lactam Resistance

Rationale The pathobiology of Staphylococcus aureus in non-cystic fibrosis bronchiectasis (nCFB) is poorly defined. When present at high density or "inoculum", some methicillin-sensitive S. aureus (MSSA) can inefficiently degrade anti-Staphylococcal beta-lactam antibiotics via BlaZ penicillinases (termed, the inoculum effect). Given the high burden of organisms in bronchiectatic airways, this is particularly relevant. Objectives Drawing from a prospectively-collected biobank, we sought to understand the prevalence, natural history, potential for transmission, and antibiotic resistance profiles amongst nCFB-derived MSSA isolates. Methods All individuals attending a regional consultancy nCFB clinic with sputum collected between 1981-2017 were considered, and those with ≥1 S. aureus-positive culture comprised the cohort. Each individual's most recent biobank isolate was subjected to whole genome sequencing (including the blaZ gene), antibacterial susceptibility testing, and comparative beta-lactam testing at standard (5 x 105CFU/mL) and high (5 x 107CFU/mL) inoculum to assess for the inoculum, and pronounced inoculum effect (IE and pIE, respectively). Results Seventy-four of 209 (35.4%) individuals had ≥1 sputum sample(s) with S. aureus (68 MSSA, 6 MRSA). Those with S. aureus infection were more likely to be female. Amongst 60/74 MSSA isolates subjected to WGS, no evidence of transmission was identified, although specific MLST types were prevalent including ST-1, ST-15, ST-30, and ST-45. Antibiotic resistance was uncommon except for macrolides (~20%). Amongst the 60 MSSA, prevalence of IE and pIE, respectively, were observed to be drug specific; meropenem (0%, 0%), cefepime (3%, 5%), ceftazidime (8%, 0%), cloxacillin (12%, 0%), cefazolin (23%, 0%) and piperacillin-tazobactam (37%, 17%). The cefazolin IE associated with blaZ type A (p<0.01) and ST-30 (p<0.01), whereas the piperacillin-tazobactam IE associated with type C blaZ (p<0.001) and ST-15 (p<0.05). Conclusions S. aureus infection was common, although no evidence of transmission was apparent in our nCFB cohort. While routine susceptibility testing did not identify significant resistance, inoculum-related resistance was found to be relevant for commonly used nCFB antibiotics including cefazolin and piperacillin-tazobactam. Given previous associations between IEs and negative patient outcomes, further work is warranted to understand how this phenotype impacts nCFB disease progression.

Epidemiology, evolution and cryptic susceptibility of methicillin-resistant Staphylococcus aureus in China: a whole-genome-based survey

OBJECTIVES

The aim of this study was to investigate the genomic epidemiology of methicillin-resistant Staphylococcus aureus (MRSA) in China to identify predominant lineages and their associations with clinical data and antimicrobial resistance profiles.

METHODS

We performed a national prevalence study of patients with S. aureus infections in 22 tertiary hospitals in China from 2015 to 2017. Clinical data from patients and the antimicrobial phenotypes were collected for each isolate. Genome sequencing was performed on a proportion of isolates and a phylogenetic analysis was undertaken. Genotypic and phenotypic β-lactam susceptibilities were compared.

RESULTS

A total of 1900 patients with S. aureus infections were included, of which 40% involved MRSA. Community-associated MRSA (CA-MRSA) infections were 24% of the total isolates. Genomic data showed that more than three-quarters of the MRSA were from three dominant lineages CC239 (25%, 116/471), CC5 (21%, 96/471) and CC59 (33%, 154/471) with CC59 accounting for more than half of the CA-MRSA isolates. Penicillin susceptibility genomic features were observed in 53% (251/470) of MRSA, including almost all of the CC59 (152/154) lineage, and 96% (242/251) of these isolates demonstrated in vitro susceptibility to penicillin or amoxicillin combined with clavulanic acid. Phylogenetic analysis indicated that the CC59 lineage can be divided into six lineages with all Asian CC59 isolates likely arising from an ancestral Mainland China lineage.

CONCLUSIONS

This study showed a high prevalence of CA-MRSA in China, largely due to the widespread presence of CC59. As almost all isolates in this lineage possess genetic variants leading to increased β-lactam susceptibility, we suggest that to improve antibiotic stewardship combinations of penicillins and β-lactamase inhibitors should be included in the antibiotic susceptibility testing panels used to inform treatment decisions and research undertaken on this combination therapy.

A Test for the Rapid Detection of the Cefazolin Inoculum Effect in Methicillin-Susceptible Staphylococcus aureus

The cefazolin inoculum effect (CzIE) has been associated with therapeutic failures and mortality in invasive methicillin-susceptible Staphylococcus aureus (MSSA) infections. A diagnostic test to detect the CzIE is not currently available. We developed a rapid (∼3 h) CzIE colorimetric test to detect staphylococcal-β-lactamase (BlaZ) activity in supernatants after ampicillin induction. The test was validated using 689 bloodstream MSSA isolates recovered from Latin America and the United States. The cefazolin MIC determination at a high inoculum (10⁷ CFU/ml) was used as a reference standard (cutoff ≥16 μg/ml). All isolates underwent genome sequencing. A total of 257 (37.3%) of MSSA isolates exhibited the CzIE by the reference standard method. The overall sensitivity and specificity of the colorimetric test was 82.5% and 88.9%, respectively. Sensitivity in MSSA isolates harboring type A BlaZ (the most efficient enzyme against cefazolin) was 92.7% with a specificity of 87.8%. The performance of the test was lower against type B and C enzymes (sensitivities of 53.3% and 72.3%, respectively). When the reference value was set to ≥32 μg/ml, the sensitivity for isolates carrying type A enzymes was 98.2%. Specificity was 100% for MSSA lacking blaZ The overall negative predictive value ranged from 81.4% to 95.6% in Latin American countries using published prevalence rates of the CzIE. MSSA isolates from the United States were genetically diverse, with no distinguishing genomic differences from Latin American MSSA, distributed among 18 sequence types. A novel test can readily identify most MSSA isolates exhibiting the CzIE, particularly those carrying type A BlaZ. In contrast to the MIC determination using high inoculum, the rapid test is inexpensive, feasible, and easy to perform. After minor validation steps, it could be incorporated into the routine clinical laboratory workflow.

Cefazolin Inoculum Effect and Methicillin-Susceptible Staphylococcus aureus Osteoarticular Infections in Children

Select methicillin-susceptible Staphylococcus aureus (MSSA) strains may produce β-lactamases with affinity for first-generation cephalosporins (1GCs). In the setting of a high inoculum, these β-lactamases may promote the cleavage of 1GCs, a phenomenon known as the cefazolin inoculum effect (CzIE). We evaluated the prevalence and impact of CzIE on clinical outcomes among MSSA acute hematogenous osteomyelitis (AHO) cases. MSSA AHO isolates obtained from two children's hospitals between January 2011 and December 2018 were procured through ongoing surveillance studies. Isolates were tested for CzIE via a broth macrodilution assay using an inoculum of 10⁷ CFU/ml; CzIE was defined as a cefazolin MIC of ≥16 μg/ml. Isolates were characterized by accessory gene regulator group (agr). The progression from acute to chronic osteomyelitis was considered an important outcome. A total of 250 cases with viable isolates were included. Notably, 14.4% of isolates exhibited CzIE with no observed temporal trend; and 4% and 76% of patients received a 1GC as an empirical and definitive therapy, respectively. CzIE isolates were more often resistant to clindamycin, belonged to agrIII, and associated with the development of chronic osteomyelitis. In multivariable analyses, agrIII, multiple surgical debridements, delayed source control, and CzIE were independently associated with progression to chronic osteomyelitis. A higher rate of chronic osteomyelitis was observed with CzIE isolates regardless of definitive antibiotic choice. CzIE is exhibited by 14.4% of MSSA AHO isolates in children. CzIE is independently associated with progression to chronic osteomyelitis in cases of AHO irrespective of final antibiotic choice. These data suggest that negative outcomes reported with CzIE may more accurately reflect strain-dependent virulence factors rather than true antibiotic failure.

Novel Insights into the Classification of Staphylococcal β-Lactamases in Relation to the Cefazolin Inoculum Effect

Cefazolin has become a prominent therapy for methicillin-susceptible Staphylococcus aureus (MSSA) infections. However, an important concern is the cefazolin inoculum effect (CzIE), a phenomenon mediated by staphylococcal β-lactamases. Four variants of staphylococcal β-lactamases have been described based on serological methodologies and limited sequence information. Here, we sought to reassess the classification of staphylococcal β-lactamases and their correlation with the CzIE. We included a large collection of 690 contemporary bloodstream MSSA isolates recovered from Latin America, a region with a high prevalence of the CzIE. We determined cefazolin MICs at standard and high inoculums by broth microdilution. Whole-genome sequencing was performed to classify the β-lactamase in each isolate based on the predicted full sequence of BlaZ. We used the classical schemes for β-lactamase classification and compared it to BlaZ allotypes found in unique sequences using the genomic information. Phylogenetic analyses were performed based on the BlaZ and core-genome sequences. The overall prevalence of the CzIE was 40%. Among 641 genomes, type C was the most predominant β-lactamase (37%), followed by type A (33%). We found 29 allotypes and 43 different substitutions in BlaZ. A single allotype, designated BlaZ-2, showed a robust and statistically significant association with the CzIE. Two other allotypes (BlaZ-3 and BlaZ-5) were associated with a lack of the CzIE. Three amino acid substitutions (A9V, E112A, and G145E) showed statistically significant association with the CzIE (P = <0.01). CC30 was the predominant clone among isolates displaying the CzIE. Thus, we provide a novel approach to the classification of the staphylococcal β-lactamases with the potential to more accurately identify MSSA strains exhibiting the CzIE.

The increased frequency of methicillin-resistant Staphylococcus aureus with low MIC of beta-lactam antibiotics isolated from hospitalized patients

Methicillin-resistant Staphylococcus aureus (MRSA) causes severe infectious diseases and can be life-threatening in healthcare-settings. MRSA is classified into health-care associated (HA)-MRSA strains and community acquired (CA)-MRSA strains based on genotype and phenotype. CA-MRSA has been reported to show the lower minimal inhibitory concentration (MIC) of some antibiotics as compared to HA-MRSA. Recently, the prevalence of CA-MRSA has been increased in worldwide. CA-MRSA is isolated not only from the healthy individuals in a community but also from the patients in healthcare settings. However, the changing trend in frequency of HA-MRSA and CA-MRSA in the hospital setting is not clear. Therefore, we analyzed the trend of MIC to speculate the frequency of HA-MRSA and CA-MRSA in the facility. Moreover, gene mutations were evaluated on resistant gene loci with next generation sequencer. The frequency of strains with low MIC of beta-lactam antibiotics was gradually increased in isolated MRSA strains from the hospitalized patients. Whole genome analysis revealed the frequency of gene mutation was also decreased in some resistant loci, such as blaZ and blaR1. These findings highlight the changing trend of MRSA strains isolated from hospitalized patients.

Genomic identification of cryptic susceptibility to penicillins and β-lactamase inhibitors in methicillin-resistant Staphylococcus aureus

Antibiotic resistance in bacterial pathogens threatens the future of modern medicine. One such resistant pathogen is methicillin-resistant Staphylococcus aureus (MRSA), which is resistant to nearly all β-lactam antibiotics, limiting treatment options. Here, we show that a significant proportion of MRSA isolates from different lineages, including the epidemic USA300 lineage, are susceptible to penicillins when used in combination with β-lactamase inhibitors such as clavulanic acid. Susceptibility is mediated by a combination of two different mutations in the mecA promoter region that lowers mecA-encoded penicillin-binding protein 2a (PBP2a) expression, and in the majority of isolates by either one of two substitutions in PBP2a (E246G or M122I) that increase the affinity of PBP2a for penicillin in the presence of clavulanic acid. Treatment of S. aureus infections in wax moth and mouse models shows that penicillin/β-lactamase inhibitor susceptibility can be exploited as an effective therapeutic choice for 'susceptible' MRSA infection. Finally, we show that isolates with the PBP2a E246G substitution have a growth advantage in the presence of penicillin but the absence of clavulanic acid, which suggests that penicillin/β-lactamase susceptibility is an example of collateral sensitivity (resistance to one antibiotic increases sensitivity to another). Our findings suggest that widely available and currently disregarded antibiotics could be effective in a significant proportion of MRSA infections.

The Cefazolin Inoculum Effect and the Presence of type A blaZ Gene according to agr Genotype in Methicillin-Susceptible Staphylococcus aureus Bacteremia

BACKGROUND

Recent data suggests the inoculum effect of methicillin-susceptible Staphylococcus aureus (MSSA) against beta-lactam antibiotics and their association with functionality or genotypic variation of agr locus.

METHODS

MSSA blood isolates were collected at a tertiary care hospital in Korea from June 2014 to December 2017. The functionality of the agr operon was measured by δ-hemolysin assays. Multiplex PCR was performed to determine the agr genotype. The cefazolin minimum inhibitory concentrations (MICs) at a high inoculum concentration (~5 × 10⁷ CFU/ml) were compared to the MICs at a standard inoculum concentration (~5 × 10⁵ CFU/ml) to identify strains with the cefazolin inoculum effect (CIE). The DNA sequencing of blaZ gene was performed to classify the blaZ genotype.

RESULTS

Among the 195 MSSA blood isolates, agr genotype I was most common (68.2%), followed by type III (16.4%), type IV (9.2%), and type II (6.2%). Sixty-seven (34.3%) MSSA isolates had dysfunctional agr, but neither CIE nor blaZ genotype was associated with dysfunctional agr. The MSSA with agr type III genotype exhibited significantly higher CIE positivity (agr III 43.8% vs. non-agr III 5.5%, P <0.01) and erythromycin/clindamycin resistance. In the subgroup analysis of type A blaZ possessing MSSA, almost all of the agr III MSSA isolates exhibited CIE, while only 20% of non-agr III isolates had CIE (P <0.01).

CONCLUSION

In MSSA blood isolates, CIE might be associated with agr genotype rather than with dysfunctional agr.

Inoculum effect of methicillin-susceptible Staphylococcus aureus against broad-spectrum beta-lactam antibiotics

Scarce information concerning the inoculum effect (InE) of methicillin-susceptible Staphylococcus aureus (MSSA) against broad-spectrum β-lactam antibiotics is available. We investigated the InE of MSSA against ceftriaxone, cefepime, meropenem, ampicillin/sulbactam and piperacillin/tazobactam. The bacteraemic MSSA isolates were collected at ten Korean general hospitals from Sep 2013 to Mar 2015. The InE was defined if MICs of antibiotics at high inoculum (HI, ~5 × 10⁷ CFU/ml) increased beyond the susceptible range compared to those at standard inoculum (SI, ~5 × 10⁵ CFU/ml). All isolates were sequenced for blaZ gene typing. Among 302 MSSA isolates, 254 (84.1%) were positive for blaZ; types A, B, C and D were 13.6%, 26.8%, 43.4% and 0.3%, respectively. Mean HI MICs of all tested antibiotics were significantly increased and increases in HI MIC of piperacillin/tazobactam (HI, 48.14 ± 4.08 vs. SI, 2.04 ± 0.08 mg/L, p < 0.001) and ampicillin/sulbactam (HI, 24.15 ± 1.27 vs. SI, 2.79 ± 0.11 mg/L, p < 0.001) were most prominent. No MSSA isolates exhibited meropenem InE, and few isolates exhibited cefepime (0.3%) and ceftriaxone (2.3%) InE, whereas 43.0% and 65.9% of MSSA isolates exhibited piperacillin/tazobactam and ampicillin/sulbactam InE, respectively. About 93% of type C blaZ versus 45% of non-type C exhibited ampicillin/sulbactam InE (p < 0.001) and 88% of type C blaZ versus 9% of non-type C exhibited piperacillin/tazobactam InE (p < 0.001). A large proportion of MSSA clinical isolates, especially those positive for type C blaZ, showed marked ampicillin/sulbactam InE and piperacillin/tazobactam.

The Cefazolin Inoculum Effect Is Associated With Increased Mortality in Methicillin-Susceptible Staphylococcus aureus Bacteremia

Background

Recent studies have favored the use of cefazolin over nafcillin for the treatment of methicillin-susceptible Staphylococcus aureus (MSSA) bacteremia. The clinical influence of the cefazolin inoculum effect (CzIE) in the effectiveness of cephalosporins for severe MSSA infections has not been evaluated.

Methods

We prospectively included patients from 3 Argentinian hospitals with S. aureus bacteremia. Cefazolin minimum inhibitory concentrations (MICs) were determined at standard (10⁵ colony-forming units [CFU]/mL) and high (10⁷ CFU/mL) inoculum. The CzIE was defined as an increase of MIC to ≥16 µg/mL when tested at high inoculum. Whole-genome sequencing was performed in all isolates.

Results

A total of 77 patients, contributing 89 MSSA isolates, were included in the study; 42 patients (54.5%) had isolates with the CzIE. In univariate analysis, patients with MSSA exhibiting the CzIE had increased 30-day mortality (P = .034) and were more likely to have catheter-associated or unknown source of bacteremia (P = .033) compared with patients infected with MSSA isolates without the CzIE. No statistically significant difference between the groups was observed in age, clinical illness severity, place of acquisition (community vs hospital), or presence of endocarditis. The CzIE remained associated with increased 30-day mortality in multivariate analysis (risk ratio, 2.65; 95% confidence interval, 1.10–6.42; P = .03). MSSA genomes displayed a high degree of heterogeneity, and the CzIE was not associated with a specific lineage.

Conclusions

In patients with MSSA bacteremia where cephalosporins are used as firstline therapy, the CzIE was associated with increased 30-day mortality. Clinicians should be cautious when using cefazolin as firstline therapy for these infections.

Inoculum effect of high concentrations of methicillin-susceptible Staphylococcus aureus on the efficacy of cefazolin and other beta-lactams

The existence of a cefazolin inoculum effect (InE) of methicillin-susceptible Staphylococcus aureus (MSSA), which is speculated to be a reason for cefazolin treatment failure in MSSA infections, is controversial. In Japan, although cefazolin is one of the therapeutic choices for patients with MSSA infection, there are few reports of this effect. Additionally, the association between InE and blaZ type in beta-lactams other than cefazolin has not been well documented. In this study, we confirmed an MSSA InE in several beta-lactams, including cefazolin, and its relationship with blaZ, using 52 MSSA isolates from blood cultures. Three isolates (5.8%) that possessed type A blaZ showed a pronounced cefazolin InE. Five isolates (9.6%) showed pronounced InE with sulbactam/ampicillin; four isolates had type C blaZ and one had type A blaZ. However, we confirmed InE in MSSA isolates with blaZ not only type A and C but also B and D. For cefotaxime, ceftriaxone, imipenem, and meropenem, regardless of the presence of blaZ, we did not observe a significant increase in MICs at a high inoculum of MSSA. Hence, our results suggest that the above four beta-lactams are good alternatives to cefazolin if InE leads to treatment failure in a patient.

Efficacy of Ceftaroline against Methicillin-Susceptible Staphylococcus aureus Exhibiting the Cefazolin High-Inoculum Effect in a Rat Model of Endocarditis

Certain Staphylococcus aureus strains exhibit an inoculum effect (InE) with cefazolin (CFZ) that has been associated with therapeutic failures in high-inoculum infections. We assessed the in vitro activities of ceftaroline (CPT), CFZ, and nafcillin (NAF) against 17 type A β-lactamase (βla)-producing, methicillin-susceptible S. aureus (MSSA) strains, including the previously reported TX0117, which exhibits the CFZ InE, and its βla-cured derivative, TX0117c. Additionally, we determined the pharmacokinetics of CPT in rats after single intramuscular doses of 20 and 40 mg/kg of body weight and evaluated the activities of CPT (40 mg/kg every 8 h [q8h]), CFZ, and NAF against TX0117 and TX0117c in a rat model of infective endocarditis. No InE was observed for CPT or NAF, whereas a marked InE was detected for CFZ (MIC, 8 to ≥128 μg/ml). CPT and NAF treatment against TX0117 resulted in mean bacterial counts of 2.3 and 2.1 log10 CFU/g in vegetations, respectively, compared to a mean of 5.9 log10 CFU/g in the CFZ-treated group (CPT and NAF versus CFZ, P = 0.001; CPT versus NAF, P = 0.9830). Both CFZ and CPT were efficacious against the βla-cured derivative, TX0117c, compared to time zero (t0) (P = <0.0001 and 0.0015, respectively). Our data reiterate the in vivo consequences of the CFZ InE and show that CPT is not affected by this phenomenon. CPT might be considered for high-inoculum infections caused by MSSA exhibiting the CFZ InE.

Association between Type A blaZ Gene Polymorphism and Cefazolin Inoculum Effect in Methicillin-Susceptible Staphylococcus aureus

Some proportion of type A blaZ gene-positive methicillin-susceptible Staphylococcus aureus strains exhibit the cefazolin inoculum effect (CIE). The type A blaZ gene was divided into two groups by single nucleotide polymorphisms (SNPs) at Ser226Pro and Cys229Tyr. The median cefazolin MICs at a high inoculum concentration were 5.69 μg/ml for the Ser-Cys group and 40.32 μg/ml for the Pro-Tyr group (P = 0.01). The SNPs at codons 226 and 229 in the amino acid sequence encoded by the blaZ gene were closely associated with the CIE.

Characteristics of cefazolin inoculum effect-positive methicillin-susceptible staphylococcus aureus infection in a multicentre bacteraemia cohort

Cefazolin treatment failure has been observed in high-inoculum infections caused by methicillin-susceptible Staphylococcus aureus (MSSA) with a cefazolin inoculum effect (CIE). However, data on the characteristics and risk factors for the acquisition of CIE-positive MSSA infection are scarce. CIE positivity was measured as an MIC ≥ 16 μg/ml with a high inoculum (∼5 × 10⁷ CFU/ml). The blaZ gene type was assessed through sequence analysis. The clinical characteristics and risk factors for the acquisition of CIE-positive MSSA infection were assessed. The association between the antimicrobial susceptibility profile and CIE positivity was evaluated. A total of 303 MSSA bacteraemia cases and their corresponding isolates were collected from ten hospitals: 61 (20.1 %) isolates showed a positive CIE; 254 (83.8 %) were positive for the blaZ gene. No significant association was found between CIE positivity and the site of infection. Metastatic cancer (aOR 2.86, 95 % CI, 1.10-7.48) and recent (≤1 month) close contact with a chronically ill patient (aOR 4.69, 95 % CI, 1.76-12.50) were identified as significant risk factors for CIE-positive MSSA infection through multivariate analyses. Resistances to clindamycin (OR 3.55, 95 % CI, 1.62-7.80) and erythromycin (OR 5.00, 95 % CI, 2.50-9.99) were associated with CIE positivity, presenting high specificity (92.9 %) and a negative predictive value (82.3 %). CIE-positive MSSA constituted approximately one-fifth of MSSA bacteraemia cases. Although CIE positivity was not clinically discernible, CIE positivity was associated with clindamycin or erythromycin susceptibility. Therefore, our findings suggest that cefazolin can be used in the treatment of high-inoculum MSSA infection if the isolates are susceptible to clindamycin or erythromycin.

Old Drugs To Treat Resistant Bugs: Methicillin-Resistant Staphylococcus aureus Isolates with mecC Are Susceptible to a Combination of Penicillin and Clavulanic Acid

β-Lactam resistance in methicillin-resistant Staphylococcus aureus (MRSA) is mediated by the expression of an alternative penicillin-binding protein 2a (PBP2a) (encoded by mecA) with a low affinity for β-lactam antibiotics. Recently, a novel variant of mecA, known as mecC, was identified in MRSA isolates from both humans and animals. In this study, we demonstrate that mecC-encoded PBP2c does not mediate resistance to penicillin. Rather, broad-spectrum β-lactam resistance in MRSA strains carrying mecC (mecC-MRSA strains) is mediated by a combination of both PBP2c and the distinct β-lactamase encoded by the blaZ gene of strain LGA251 (blaZLGA251), which is part of mecC-encoding staphylococcal cassette chromosome mec (SCCmec) type XI. We further demonstrate that mecC-MRSA strains are susceptible to the combination of penicillin and the β-lactam inhibitor clavulanic acid in vitro and that the same combination is effective in vivo for the treatment of experimental mecC-MRSA infection in wax moth larvae. Thus, we demonstrate how the distinct biological differences between mecA- and mecC-encoded PBP2a and PBP2c have the potential to be exploited as a novel approach for the treatment of mecC-MRSA infections.

Prevalence of blaZ gene types and the cefazolin inoculum effect among methicillin-susceptible Staphylococcus aureus blood isolates and their association with multilocus sequence types and clinical outcome

Cefazolin treatment failures have been described for bacteraemia caused by methicillin-susceptible Staphylococcus aureus (MSSA) with type A β-lactamase and inoculum effect (InE). We investigated the prevalence of blaZ (β-lactamase) gene types and a cefazolin InE among MSSA blood isolates in South Korea and evaluated their association with specific genotypes. The clinical impact of the cefazolin InE was also evaluated. A total of 220 MSSA isolates were collected from a prospective cohort study of S. aureus bacteraemia. A pronounced InE with cefazolin was defined as a ≥4-fold increase in the minimum inhibitory concentration (MIC) between a standard and high inoculum, resulting in a non-susceptible MIC. Sequencing of blaZ and multilocus sequence typing (MLST) were performed. Clinical outcomes were assessed in 77 patients treated with cefazolin. The blaZ gene was detected in 92 % of the 220 MSSA isolates. Type C β-lactamase was the most common (53 %), followed by type B (20 %) and type A (17 %). Certain genotypes were significantly associated with specific β-lactamase types (notably, ST30 and type A β-lactamase). A pronounced cefazolin InE was observed in 13 % of isolates. Most of these (79 %) expressed type A β-lactamase and ST30 was the predominant (55 %) clone amongst them. Cefazolin treatment failure was not observed in patients infected with strains exhibiting a pronounced InE. These strains had no impact on other clinical outcomes. In conclusion, the prevalence of a pronounced InE with cefazolin could be dependent upon distributions of MSSA genotypes. Cefazolin can likely be used for the treatment of MSSA bacteraemia (except endocarditis), without consideration of an InE.

Real-time PCR assay for detection of blaZ genes in Staphylococcus aureus clinical isolates

The Clinical and Laboratory Standards Institute recommends consideration of blaZ gene testing for cases of serious Staphylococcus aureus infection. Conventional PCR methods have demonstrated superior sensitivity and specificity to phenotypic tests. To our knowledge, this is the first description of real-time PCR detection of the blaZ gene.

Novel mutations in penicillin-binding protein genes in clinical Staphylococcus aureus isolates that are methicillin resistant on susceptibility testing, but lack the mec gene

Objectives

Methicillin-resistant Staphylococcus aureus (MRSA) is an important global health problem. MRSA resistance to β-lactam antibiotics is mediated by the mecA or mecC genes, which encode an alternative penicillin-binding protein (PBP) 2a that has a low affinity to β-lactam antibiotics. Detection of mec genes or PBP2a is regarded as the gold standard for the diagnosis of MRSA. We identified four MRSA isolates that lacked mecA or mecC genes, but were still phenotypically resistant to pencillinase-resistant β-lactam antibiotics.

Methods

The four human S. aureus isolates were investigated by whole genome sequencing and a range of phenotypic assays.

Results

We identified a number of amino acid substitutions present in the endogenous PBPs 1, 2 and 3 that were found in the resistant isolates but were absent in closely related susceptible isolates and which may be the basis of resistance. Of particular interest are three identical amino acid substitutions in PBPs 1, 2 and 3, occurring independently in isolates from at least two separate multilocus sequence types. Two different non-conservative substitutions were also present in the same amino acid of PBP1 in two isolates from two different sequence types.

Conclusions

This work suggests that phenotypically resistant MRSA could be misdiagnosed using molecular methods alone and provides evidence of alternative mechanisms for β-lactam resistance in MRSA that may need to be considered by diagnostic laboratories.

In Vivo Effects of Cefazolin, Daptomycin, and Nafcillin in Experimental Endocarditis with a Methicillin-Susceptible Staphylococcus aureus Strain Showing an Inoculum Effect against Cefazolin

Several reports have implicated the inoculum effect that some strains of type A beta-lactamase (Bla)-producing, methicillin-susceptible Staphylococcus aureus (MSSA) show against cefazolin as the cause for clinical failures in certain serious deep-seated infections. Here, using a previously reported MSSA strain displaying this phenotype (TX0117), we obtained a Bla-cured derivative (TX0117c) with a combination of novobiocin and high temperature. Both isolates were then used in a rat endocarditis model and treated with cefazolin, nafcillin, and daptomycin, given to simulate human dosing. Animals were treated for 3 days and either sacrificed at 24 h after the last antibiotic dose (standard group) or left untreated for an additional 3 days (relapse group). With TX0117 in the standard treatment group, daptomycin and nafcillin were both significantly better than cefazolin in reducing CFU/g of vegetations, achieving mean log₁₀ reductions compared to levels in untreated rats of 7.1, 5.3, and 1.8, respectively (cefazolin versus daptomycin, P < 0.0001; cefazolin versus nafcillin, P = 0.005; daptomycin versus nafcillin, P = 0.053). In addition, cefazolin was significantly more effective in reducing vegetation titers of TX0117c than of TX0117 (mean log₁₀ reduction of 1.4 versus 5.5, respectively; P = 0.0001). Similar results were observed with animals in the relapse group. Thus, these data show that there can be an in vivo consequence of the in vitro inoculum effect that some MSSA strains display against cefazolin and indicate a specific role for Bla production using a Bla-cured derivative strain against which cefazolin regained both in vitro and in vivo activity.

Cefazolin high-inoculum effect in methicillin-susceptible Staphylococcus aureus from South American hospitals

OBJECTIVES

Clinical failures with cefazolin have been described in high-inoculum infections caused by methicillin-susceptible Staphylococcus aureus (MSSA) producing type A β-lactamase. We investigated the prevalence of the cefazolin inoculum effect (InE) in MSSA from South American hospitals, since cefazolin is used routinely against MSSA due to concerns about the in vivo efficacy of isoxazolyl penicillins.

METHODS

MSSA isolates were recovered from bloodstream (n = 296) and osteomyelitis (n = 68) infections in two different multicentre surveillance studies performed in 2001-02 and 2006-08 in South American hospitals. We determined standard-inoculum (10(5)cfu/mL) and high-inoculum (10(7) cfu/mL) cefazolin MICs. PFGE was performed on all isolates that exhibited a cefazolin InE. Multilocus sequence typing (MLST) and sequencing of part of blaZ were performed on representative isolates.

RESULTS

The overall prevalence of the cefazolin InE was 36% (131 isolates). A high proportion (50%) of MSSA isolates recovered from osteomyelitis infections exhibited the InE, whereas it was observed in 33% of MSSA recovered from bloodstream infections. Interestingly, Ecuador had the highest prevalence of the InE (45%). Strikingly, 63% of MSSA isolates recovered from osteomyelitis infections in Colombia exhibited the InE. MLST revealed that MSSA isolates exhibiting the InE belonged to diverse genetic backgrounds, including ST5, ST8, ST30 and ST45, which correlated with the prevalent methicillin-resistant S. aureus clones circulating in South America. Types A (66%) and C (31%) were the most prevalent β-lactamases.

CONCLUSIONS

Our results show a high prevalence of the cefazolin InE associated with type A β-lactamase in MSSA isolates from Colombia and Ecuador, suggesting that treatment of deep-seated infections with cefazolin in those countries may be compromised.

Staphylococcus aureus extracellular vesicles carry biologically active β-lactamase

Gram-positive bacteria naturally produce extracellular vesicles. However, little is known regarding the functions of Gram-positive bacterial extracellular vesicles, especially in the bacterial community. Here, we investigated the role of Staphylococcus aureus extracellular vesicles in interbacterial communication to cope with antibiotic stress. We found that S. aureus liberated BlaZ, a β-lactamase protein, via extracellular vesicles. These extracellular vesicles enabled other ampicillin-susceptible Gram-negative and Gram-positive bacteria to survive in the presence of ampicillin. However, S. aureus extracellular vesicles did not mediate the survival of tetracycline-, chloramphenicol-, or kanamycin-susceptible bacteria. Moreover, S. aureus extracellular vesicles did not contain the blaZ gene. In addition, the heat-treated S. aureus extracellular vesicles did not mediate the survival of ampicillin-susceptible bacteria. The β-lactamase activities of S. aureus soluble and extracellular vesicle-associated BlaZ were similar, but only the extracellular vesicle-associated BlaZ was resistant to protease digestion, which suggests that the enzymatic activity of BlaZ in extracellular vesicles is largely protected by the vesicle structure. Our observations provide evidence of the important role of S. aureus extracellular vesicles in antibiotic resistance, which allows the polymicrobial community to continue to evolve and prosper against antibiotics.

A genomic portrait of the emergence, evolution, and global spread of a methicillin-resistant Staphylococcus aureus pandemic

The widespread use of antibiotics in association with high-density clinical care has driven the emergence of drug-resistant bacteria that are adapted to thrive in hospitalized patients. Of particular concern are globally disseminated methicillin-resistant Staphylococcus aureus (MRSA) clones that cause outbreaks and epidemics associated with health care. The most rapidly spreading and tenacious health-care-associated clone in Europe currently is EMRSA-15, which was first detected in the UK in the early 1990s and subsequently spread throughout Europe and beyond. Using phylogenomic methods to analyze the genome sequences for 193 S. aureus isolates, we were able to show that the current pandemic population of EMRSA-15 descends from a health-care-associated MRSA epidemic that spread throughout England in the 1980s, which had itself previously emerged from a primarily community-associated methicillin-sensitive population. The emergence of fluoroquinolone resistance in this EMRSA-15 subclone in the English Midlands during the mid-1980s appears to have played a key role in triggering pandemic spread, and occurred shortly after the first clinical trials of this drug. Genome-based coalescence analysis estimated that the population of this subclone over the last 20 yr has grown four times faster than its progenitor. Using comparative genomic analysis we identified the molecular genetic basis of 99.8% of the antimicrobial resistance phenotypes of the isolates, highlighting the potential of pathogen genome sequencing as a diagnostic tool. We document the genetic changes associated with adaptation to the hospital environment and with increasing drug resistance over time, and how MRSA evolution likely has been influenced by country-specific drug use regimens.

Identification of a β-lactamase inhibitory protein variant that is a potent inhibitor of Staphylococcus PC1 β-lactamase

β-Lactamase inhibitory protein (BLIP) binds and inhibits a diverse collection of class A β-lactamases. Widespread resistance to β-lactam antibiotics currently limits the treatment strategies for Staphylococcus infections. The goals of this study were to determine the binding affinity of BLIP for Staphylococcus aureus PC1 β-lactamase and to identify mutants that alter binding affinity. The BLIP inhibition constant (K(i)) for PC1 β-lactamase was measured at 350 nM, and isothermal titration calorimetry experiments indicated a binding constant (K(d)) of 380 nM. Twenty-three residue positions in BLIP that contact β-lactamase were randomized, and phage display was used to sort the libraries for tight binders to immobilized PC1 β-lactamase. The BLIP(K74G) mutant was the dominant clone selected, and it was found to inhibit the PC1 β-lactamase with a K(i) of 42 nM, while calorimetry indicated a K(d) of 26 nM. Molecular modeling studies suggested that BLIP binds weakly to the PC1 β-lactamase due to the presence of alanine at position 104 of PC1. This position is occupied by glutamate in the TEM-1 enzyme, where it forms a salt bridge with the BLIP residue Lys74 that is important for the stability of the complex. This hypothesis was confirmed by showing that the PC1(A104E) enzyme binds BLIP with 15-fold greater affinity than wild-type PC1 β-lactamase. Kinetic measurements indicated similar association rates for all complexes with variation in affinity due to altered dissociation rate constants, suggesting that changes in short-range interactions are responsible for the altered binding properties of the mutants.

Determination of an inoculum effect with various cephalosporins among clinical isolates of methicillin-susceptible Staphylococcus aureus

Using 98 clinical methicillin-susceptible Staphylococcus aureus isolates of known beta-lactamase (Bla) type, we found a pronounced inoculum effect for cephalexin (mostly Bla type A and C strains), a mild inoculum effect for cephalothin (especially types B and C), and no inoculum effects for ceftriaxone and cefuroxime. Ceftobiprole showed the lowest MICs at a high inoculum but with a slight increase for Bla-positive versus Bla-negative strains. Since a potential therapeutic effect associated with a cephalosporin inoculum effect has been described, further studies are warranted.

Inoculum effect with cefazolin among clinical isolates of methicillin-susceptible Staphylococcus aureus: frequency and possible cause of cefazolin treatment failure

Methicillin (meticillin)-susceptible Staphylococcus aureus (MSSA) strains producing large amounts of type A beta-lactamase (Bla) have been associated with cefazolin failures, but the frequency and impact of these strains have not been well studied. Here we examined 98 MSSA clinical isolates and found that 26% produced type A Bla, 15% type B, 46% type C, and none type D and that 13% lacked blaZ. The cefazolin MIC(90) was 2 microg/ml for a standard inoculum and 32 microg/ml for a high inoculum, with 19% of isolates displaying a pronounced inoculum effect (MICs of >or=16 microg/ml with 10(7) CFU/ml) (9 type A and 10 type C Bla producers). At the high inoculum, type A producers displayed higher cefazolin MICs than type B or C producers, while type B and C producers displayed higher cefamandole MICs. Among isolates from hemodialysis patients with MSSA bacteremia, three from the six patients who experienced cefazolin failure showed a cefazolin inoculum effect, while none from the six patients successfully treated with cefazolin showed an inoculum effect, suggesting an association between these strains and cefazolin failure (P = 0.09 by Fisher's exact test). In summary, 19% of MSSA clinical isolates showed a pronounced inoculum effect with cefazolin, a phenomenon that could explain the cases of cefazolin failure previously reported for hemodialysis patients with MSSA bacteremia. These results suggest that for serious MSSA infections, the presence of a significant inoculum effect with cefazolin could be associated with clinical failure in patients treated with this cephalosporin, particularly when it is used at low doses.

Phage typing, PCR amplification for mecA gene, and antibiotic resistance patterns as epidemiologic markers in nosocomial outbreaks of methicillin resistant Staphylococcus aureus

Staphylococcus aureus is one of the major causes of community and hospital-acquired infections. Bacteriophage considered as a major risk factor acquires S. aureus new virulence genetic elements. A total number of 119 S. aureus isolated from different specimens obtained from (RKH) were distinguished by susceptibility to 19 antimicrobial agents, phage typing, and PCR amplification for mecA gene. All of MRSA isolates harbored mecA gene, except three unique isolates. The predominant phage group is belonging to the (mixed group). Phage group (II) considered as an epidemiological marker correlated to β-lactamase hyper producer isolates. MRSA isolates indicated high prevalence of phage group (II) with highly increase for phage types (Ø3A), which were correlated to the skin. Phage types (Ø80/Ø81) played an important roll in Community Acquired Methicillin Resistant S. aureus (CAMRSA). Three outpatients MRSA isolates had low multiresistance against Bacitracin (Ba) and Fusidic acid (FD), considered as CAMRSA isolates. It was detected that group I typed all FD-resistant MSSA isolates. Phage groups (M) and (II) were found almost to be integrated for Gentamycin (GN) resistance especially phage type (Ø95) which relatively increased up to 20% in MRSA. Tetracycline (TE) resistant isolates typed by groups (II) and (III) in MSSA. Only one isolate resistant to Sulphamethoxazole/Trimethoprim (SXT) was typed by (III/V) alone in MSSA. MRSA isolates resistant to Chloramphenicol (C) and Ba were typed by all groups except (V). It could be concluded that (PERSA) S. aureus isolates from the wound that originated and colonized, and started to build up multi-resistance against the topical treatment antibiotics. In this study, some unique sporadic isolates for both MRSA and MSSA could be used as biological, molecular and epidemiological markers such as prospective tools.

Comparison of tests for detection of beta-lactamase-producing staphylococci

Penicillin resistance identification tests are important in veterinary medicine. Six enzyme assays and a PCR test were compared for the detection of beta-lactamase production or the beta-lactamase gene in 175 staphylococcal isolates. We conclude that the PCR test and two nitrocefin-based assays can be recommended for routine clinical use.

Diversity and evolution of blaZ from Staphylococcus aureus and coagulase-negative staphylococci

OBJECTIVES

To elucidate the diversity and evolutionary history of plasmid- and chromosomally-located blaZ, to detect indications of frequent exchange of blaZ between human and bovine staphylococci and to estimate the frequency of transfer of blaZ between coagulase-negative staphylococci (CoNS) and Staphylococcus aureus of bovine origin.

METHODS

blaZ was detected in 143 strains of penicillin-resistant S. aureus and CoNS from five Danish cattle herds (n = 25/23), random CoNS isolates from Denmark (n = 37), a collection of S. aureus from six different countries (n = 52), humans in Denmark (n = 3) and beta-lactamase control strains (n = 3). The sequence was determined in 105 strains and compared to published sequences by pairwise and multiple alignments. Maximum likelihood analysis was performed including bootstrap analysis. Parsimony, neighbour joining and consensus comparisons were performed for recombination. The localization of blaZ was determined by Southern blotting in 108 isolates.

RESULTS

All penicillin-resistant strains carried blaZ and showed a similar organization of blaR1 and blaZ. The blaZ gene was localized to a plasmid in only 16 of the resistant strains. Sixty-nine sequences representing 105 isolates and sequences retrieved from public databases were compared. A phylogenetic tree showed that blaZ exists in three evolutionary lines: one group was of plasmid origin, one group was of chromosomal origin and one intermediate group. Sixty-nine sequence types were demonstrated. They translated into 11 BlaZ protein types. The major types all contained strains of both human and bovine origin, and more than one Staphylococcus species, demonstrating a shared gene pool. In a comparison of S. aureus and CoNS obtained from five Danish cattle herds, the same type of blaZ was only detected in one case.

CONCLUSIONS

Results indicated a separate evolution for plasmid- and chromosomally-encoded blaZ. Although a common gene pool seems to exist among staphylococci, exchange of blaZ between strains and species is judged to be an extremely rare event.

Relapse of Type A -Lactamase-Producing Staphylococcus aureus Native Valve Endocarditis during Cefazolin Therapy: Revisiting the Issue

Our experience with a patient with methicillin-susceptible Staphylococcus aureus aortic native valve endocarditis, who had a relapse involving fever and positive blood culture results while receiving cefazolin, led us to evaluate this organism's ability to hydrolyze cefazolin at high inocula, a previously well-documented phenomenon. Analysis of the infecting strain disclosed a high minimum inhibitory concentration of cefazolin when a large inoculum was used, as well as rapid and complete cefazolin degradation, which was associated with regrowth in a time-kill experiment. DNA sequencing of the beta-lactamase gene showed that it was identical to that of the S. aureus type A beta-lactamase, known to efficiently inactivate cefazolin. A word of caution is given regarding the use of this antibiotic for treatment of endocarditis caused by this type of S. aureus isolate.

Genetic linkage between resistance to quaternary ammonium compounds and beta-lactam antibiotics in food-related Staphylococcus spp

Little is known about the occurrence of antimicrobial resistance determinants in staphylococci isolated from food and food processing industries. Quaternary ammonium compound (QAC)-resistant coagulase-negative staphylococci (CNS) isolated from food and food-processing industries were investigated for the presence of genetic determinants (qacA/B and qacC/smr) encoding resistance to the QAC benzalkonium chloride (BC), several antibiotic resistance genes, and staphylococcal insertion sequences IS257 and IS256. Six qacA/B-harboring strains were resistant to penicillin and hybridized to a blaZ probe. The qacA/B and blaZ probes hybridized to plasmids of similar size in three isolates. Molecular and genetic characterization of the 23-kb plasmid (pST6) of Staphylococcus epidermidis St.6 revealed the presence of qacB adjacent to an incomplete beta-lactamase transposon Tn552 encoding the gene cluster blaZ, blaR, and blaI. Sequence analysis of flanking regions and the intergenic region between blaZ and qacB revealed the presence of IS257 downstream of blaZ as well as sin and binR between blaZ and qacB. In the three other BC and penicillin-resistant strains, the qacA/B and blaZ genes were located on separate plasmids. A qacC harboring S. epidermidis strain (St.17) also hybridized to tetK (tetracycline resistance) and ermB (erythromycin resistance) genes. The individual genes were located on separate plasmids, suggesting no linkage between QAC and antibiotic resistance determinants. Plasmid-free Staphylococcus aureus RN4220 allowed uptake of the pST6 plasmid DNA, indicating that the resistance genes could potentially be transferred to pathogens under selective stress. In conclusion, presence of both resistance determinants could lead to co-selection during antimicrobial therapy or disinfection in hospitals or in food industries.

Effect of lactoferrin in combination with penicillin on the morphology and the physiology of Staphylococcus aureus isolated from bovine mastitis

The objective of the present study was to evaluate the therapeutic potential of bovine lactoferrin or lactoferricin in combination with penicillin G against Staphylococcus aureus. Minimal inhibitory concentrations of lactoferrin, lactoferricin, penicillin, and combinations of lactoferrin or lactoferricin with penicillin were determined for 15 S. aureus strains including several strains resistant to beta-lactam antibiotics. The fractional inhibitory concentration index indicated a synergistic effect between lactoferrin and penicillin. Combination of lactoferrin with penicillin increased the inhibitory activity of penicillin by two- to fourfold and reduced the growth rate in S. aureus strains tested, whereas the increase in the inhibitory activity of lactoferrin by penicillin was 16- to 64-fold. The addition of iron to the medium containing a combination of penicillin and lactoferrin had no effect on growth inhibition. Electron microscopy revealed that concentration below the minimal inhibitory concentrations of penicillin induced important ultrastructure alterations, which were further enhanced by the presence of lactoferrin. When S. aureus cells were grown in the presence of a combination of penicillin and lactoferrin, changes in the protein profile of the bacteria, including the disappearance of several protein bands due to the presence of lactoferrin, were observed. These data suggest that bovine lactoferrin or lactoferricin in combination with beta-lactam antibiotics can increase the antibacterial activity of these antibiotics against S. aureus resistant to antibiotics.

False-positive beta-lactamase results with Staphylococcus lugdunensis in the Vitek AutoMicrobic system

The Vitek AutoMicrobic system in combination with the Gram-positive susceptibility card detects beta-lactamase in staphylococci by utilizing penicillin as the substrate coupled with oxacillin as an inducer. The beta-lactamase activity of 21 clinical isolates and two reference strains of Staphylococcus lugdunensis was determined with this automated system and compared with a liquid nitrocefin assay after induction with oxacillin. Eight (38%) clinical isolates and the reference strain ATCC 49576 of S. lugdunensis showed production of beta-lactamase in both tests. Thirteen (62%) clinical isolates and the type strain ATCC 43809 were nitrocefin-negative. The Vitek AutoMicrobic system reported false-positive beta-lactamase results for 9 of those 13 isolates and for the type strain of S. lugdunensis. Results for disk diffusion (ampicillin) were concordant with the nitrocefin assay. With one exception, the MICs for penicillin of the nitrocefin-negative strains were in the equivocal range of 0.06-0.12 mg/l according to NCCLS. However, none of the nitrocefin-negative and Vitek-positive strains revealed any of the known staphylococcal genes for beta-lactamase as investigated by Southern hybridization, supporting the fact that false-positive beta-lactamase results may occur in the Vitek AutoMicrobic system. We conclude from our data that it may be justified to include S. lugdunensis in the quality control of Vitek cards containing beta-lactamase tests.