Emergence of high rates of antimicrobial resistance among viridans group streptococci in the United States

Antimicrobial susceptibility of Viridians Group of Streptococci isolated from infective endocarditis patients from 2018 to 2023

INTRODUCTION

Viridans Group of Streptococci (VGS) are heterogeneous alpha hemolytic Streptococci that form commensal flora in the oral cavity, upper respiratory tract, gastrointestinal tract and female genital tract and are potential pathogens that can cause serious infections like infective endocarditis and bacteremia. Penicillin or Ceftriaxone is the recommended first line agent for definitive therapy of VGS endocarditis. Alteration in penicillin binding proteins can decrease the susceptibility of VGS to penicillin and other beta-lactam agents and complicate antimicrobial therapy for serious VGS infections like Infective endocarditis. The aim of our study was to determine the antimicrobial susceptibility pattern of viridians group Streptococci isolated from blood samples of infective endocarditis patients from April 2018 to September 2023 against betalactam and other agents.

MATERIAL AND METHODS

Three sets of blood cultures with 8-10 ml per bottle were collected from suspected infective endocarditis and incubated in automated blood culture system (BACTEC from BD diagnostics). The broth from positive bottles was streaked on Blood agar, Chocolate agar and MacConkey agar plates and incubated at 37°C. Alpha hemolytic Streptococci that grew after 24-48 h were further identified and antimicrobial susceptibility determined by Vitek 2 system.

RESULTS

A total of 100 VGS was isolated from blood samples of Infective endocarditis patients. The species most commonly isolated in our study was S. sanguinis (25%) and S.mitis/oralis group (23%) followed by Non speciated Viridans Streptococci (17%). The lowest penicillin susceptibility ie. PEN MIC <0.12 μg/ml of 40% was observed in S. alactolyticus group, followed by S. mitis (52%) and S. gallolyticus (56%). S. mitis/oralis group showed the lowest susceptibilities to betalactam antibiotics among all speciated and non speciated VGS.

CONCLUSION

Infective endocarditis carries a very high mortality and morbidity and the emergence of resistance to betalactam agents like penicillin will only narrow available therapeutic options and further challenge the treatment.

Aerococcus viridans and Public Health: Oral Carriage and Antimicrobial Resistance in Stray Dogs and Cats in Algeria

The current study aimed to determine the occurrence and antimicrobial resistance of oral Aerococcus viridans in stray dogs and cats in Algeria. Oral swabs from 200 stray animals (100 dogs and 100 cats) were collected and cultured on Columbia agar medium supplemented with 5% defibrinated sheep blood. Isolates were identified using analytical profile index Rapid 20 Strep commercial kits, and antibiotic susceptibility was determined using the disk diffusion method. Of the 200 animals sampled, 34 carried A. viridans in their oral cavities, with 26 isolates (76.47%) resistant to at least 2 drugs. Multidrug resistance profiles (to more than three different antimicrobials) were observed only in cats (26.08% of isolates). More isolates were resistant to erythromycin and tetracycline (71% and 65%, respectively) than to other antimicrobials. This is the first research study in Algeria detecting antimicrobial resistance in oral A. viridans isolated from dogs and cats and highlights potential public health concerns. Clinical trials registration number: 01/2018.

Microbiological Epidemiology of Invasive Infections Due to Non-Beta-Hemolytic Streptococci, France, 2021

Non-beta-hemolytic streptococci (NBHS), also referred to as viridans streptococci, represent an underestimated cause of human invasive diseases. Their resistance to antibiotics, including beta-lactam agents, often complicate their therapeutic management. A prospective multicenter study was conducted by the French National Reference Center for Streptococci between March and April 2021 to describe the clinical and microbiological epidemiology of invasive infections due to NBHS, excluding pneumococcus. A total of 522 NBHS invasive cases were collected. Distribution among streptococcal groups was: Streptococcus anginosus (33%), Streptococcus mitis (28%), Streptococcus sanguinis (16%), Streptococcus bovis/equinus (15%), Streptococcus salivarius (8%), and Streptococcus mutans (<1%). Median age of infection was 68 years old (range <1 day to 100 years). Cases were more frequent in male patients (gender ratio M/F 2.1:1) and manifested mainly as bacteremia without focus (46%), intra-abdominal infections (18%) and endocarditis (11%). All isolates were susceptible to glycopeptides and displayed low-level inherent gentamicin resistance. All isolates of the S. bovis/equinus, S. anginosus, and S. mutans groups were susceptible to beta-lactams. Conversely, nonsusceptibility to beta-lactams was found in 31%, 28%, and 52% of S. mitis, S. salivarius, and S. sanguinis isolates, respectively. The screening for beta-lactam resistance using the recommended one unit benzylpenicillin disk screening failed to detect 21% of resistant isolates (21/99). Last, overall resistance rates to the alternative anti-streptococcal molecules clindamycin and moxifloxacin were 29% (149/522) and 1.6% (8/505), respectively. IMPORTANCE NBHS are recognized as opportunistic pathogens particularly involved in infections of the elderly and immunocompromised patients. This study underlines their importance as common causes of severe and difficult-to-treat infections such as endocarditis. Although species of the S. anginosus and S. bovis/equinus groups remain constantly susceptible to beta-lams, resistance in oral streptococci exceeds 30% and screening techniques are not fully reliable. Therefore, accurate species identification and antimicrobial susceptibility testing by MICs determination appears essential for the treatment of NBHS invasive infections, together with continued epidemiological surveillance.

Bacterial cell membranes and their role in daptomycin resistance: A review

Lipids play a major role in bacterial cells. Foremost, lipids are the primary constituents of the cell membrane bilayer, providing structure and separating the cell from the surrounding environment. This makes the lipid bilayer a prime target for antimicrobial peptides and membrane-acting antibiotics such as daptomycin. In response, bacteria have evolved mechanisms by which the membrane can be adapted to resist attack by these antimicrobial compounds. In this review, we focus on the membrane phospholipid changes associated with daptomycin resistance in enterococci, Staphylococcus aureus, and the Viridans group streptococci.

Phenotypic Adaptation to Antiseptics and Effects on Biofilm Formation Capacity and Antibiotic Resistance in Clinical Isolates of Early Colonizers in Dental Plaque

Despite the wide-spread use of antiseptics in dental practice and oral care products, there is little public awareness of potential risks associated with antiseptic resistance and potentially concomitant cross-resistance. Therefore, the aim of this study was to investigate potential phenotypic adaptation in 177 clinical isolates of early colonizers of dental plaque (Streptococcus, Actinomyces, Rothia and Veillonella spp.) upon repeated exposure to subinhibitory concentrations of chlorhexidine digluconate (CHX) or cetylpyridinium chloride (CPC) over 10 passages using a modified microdilution method. Stability of phenotypic adaptation was re-evaluated after culture in antiseptic-free nutrient broth for 24 or 72 h. Strains showing 8-fold minimal inhibitory concentration (MIC)-increase were further examined regarding their biofilm formation capacity, phenotypic antibiotic resistance and presence of antibiotic resistance genes (ARGs). Eight-fold MIC-increases to CHX were detected in four Streptococcus isolates. These strains mostly exhibited significantly increased biofilm formation capacity compared to their respective wild-type strains. Phenotypic antibiotic resistance was detected to tetracycline and erythromycin, consistent with the detected ARGs. In conclusion, this study shows that clinical isolates of early colonizers of dental plaque can phenotypically adapt toward antiseptics such as CHX upon repeated exposure. The underlying mechanisms at genomic and transcriptomic levels need to be investigated in future studies.

Multiple Deep-seated Dentofacial Abscesses Caused by Multidrug-resistant Viridans Group Streptococci

Odontogenic infections, generally caused by dental caries and periodontal disease, can result in fatal illness. We herein report a 71-year-old Japanese woman with type 2 diabetes and hemodialysis who suffered from multiple dentofacial abscesses mainly caused by multidrug-resistant Streptococcus oralis. She complained of pain and swelling of her face, with an extraoral fistula from the left cheek. Following 3 surgical debridement procedures and partial mandibulectomy, in addition to 12 weeks of antimicrobial therapy, the multiple dentofacial abscesses were ameliorated. A combination of surgical and antimicrobial treatments following an early diagnosis is essential for reducing further complications.

Evaluation of a Paradigm Shift From Intravenous Antibiotics to Oral Step-Down Therapy for the Treatment of Infective Endocarditis: A Narrative Review

IMPORTANCE

The requirement of prolonged intravenous antibiotic courses to treat infective endocarditis (IE) is a time-honored dogma of medicine. However, numerous antibiotics are now available that achieve adequate levels in the blood after oral administration to kill bacteria. Moreover, prolonged intravenous antibiotic regimens are associated with high rates of adverse events. Accordingly, recent studies of oral step-down antibiotic treatment have stimulated a reevaluation of the need for intravenous-only therapy for IE.

OBSERVATIONS

PubMed was reviewed in October 2019, with an update in February 2020, to determine whether evidence supports the notion that oral step-down antibiotic therapy for IE is associated with inferior outcomes compared with intravenous-only therapy. The search identified 21 observational studies evaluating the effectiveness of oral antibiotics for treating IE, typically after an initial course of intravenous therapy; none found such oral step-down therapy to be inferior to intravenous-only therapy. Multiple studies described an improved clinical cure rate and an improved mortality rate among patients treated with oral step-down vs intravenous-only antibiotic therapy. Three randomized clinical trials also demonstrated that oral step-down antibiotic therapy is at least as effective as intravenous-only therapy in right-sided, left-sided, or prosthetic valve IE. In the largest trial, at 3.5 years of follow-up, patients randomized to receive oral step-down antibiotic therapy had a significantly improved cure rate and mortality rate compared with those who received intravenous-only therapy.

CONCLUSIONS AND RELEVANCE

This review found ample data demonstrating the therapeutic effectiveness of oral step-down vs intravenous-only antibiotic therapy for IE, and no contrary data were identified. The use of highly orally bioavailable antibiotics as step-down therapy for IE, after clearing bacteremia and achieving clinical stability with intravenous regimens, should be incorporated into clinical practice.

Mitral Valve Replacement Under Venoarterial Extracorporeal Membrane Oxygenation Support for Severe Streptococcus Mitis Endocarditis

Streptococcus mitis is an alpha-hemolytic species of Streptococcus and is the most prevalent organism of the oral flora. In patients with dental procedures or trauma and underlying damaged heart valves, it can cause infection of the valves (endocarditis). It has a propensity to affect the left-sided heart valves (mitral and aortic valves) in non-intravenous drug users (IVDU), whereas right-sided heart valves (tricuspid and pulmonic valves) in IVDU. We describe a case of a patient who presented with mitral valve endocarditis that was treated with antibiotics. He was lost to follow-up and then presented four years after his index presentation in cardiogenic shock from severe streptococcal mitis endocarditis causing severe mitral regurgitation. His course deteriorated to the point where the patient required a valve replacement surgery under hemodynamic support. 

An intubated 7-month-old infant with a retropharyngeal abscess and multidrug-resistant Streptococcus mitis

The profile of antimicrobial resistance (ie, antibiogram) may be disparate between children and adults. An infant developed severe deep neck infection with a multidrug-resistant microbe. The microbe was more drug-resistant in children than in adults, in our hospital. Treatment of a child should be guided by the antibiogram obtained from children.

Infective endocarditis: a retrospective cohort study

BACKGROUND

Infective endocarditis (IE) is a potentially life-threatening infection of the heart's endocardial surface. Despite advances in the diagnosis and management of IE, morbidity and mortality remain high.

AIM

To characterize the demographics, bacteriology and outcomes of IE cases presenting to an Irish tertiary referral centre.

DESIGN

Retrospective cohort study.

METHODS

Patients were identified using Hospital Inpatient Enquiry and Clinical Microbiology inpatient consult data, from January 2005 to January 2014. Patients were diagnosed with IE using Modified Duke Criteria. Standard Bayesian statistics were employed for analysis and cases were compared to contemporary international registries.

RESULTS

Two hundred and two patients were diagnosed with IE during this period. Mean age 54 years. Of these, 136 (67%) were native valve endocarditis (NVE), 50 (25%) were prosthetic valve endocarditis (PVE) and 22 (11%) were cardiovascular implantable electronic device-associated endocarditis. Culprit organism was identified in 176 (87.1%) cases and Staphylococcal species were the most common (57.5%). Fifty-nine per cent of NVE required surgery compared to 66% of PVE. Mean mortality rate was 17.3%, with NVE being the lowest (12.5%) and PVE the highest (32%). Increasing age was also associated with increased mortality. Fifty-three (26.2%) patients had embolic complications.

CONCLUSIONS

This Irish cohort exhibited first-world demographic patterns comparable to those published in contemporary international literature. PVE required surgery more often and was associated with higher rates of mortality than NVE. Embolic complications were relatively common and represent important sequelae, especially in the intravenous drug user population. It is also pertinent to aggressively treat older cohorts as they were associated with increased mortality.

Endocarditis Caused by Highly Penicillin-Resistant Viridans Group Streptococci: Still Room for Vancomycin-Based Regimens

Optimal treatment options remain unknown for infective endocarditis (IE) caused by penicillin-resistant (PEN-R) viridans group streptococcal (VGS) strains. The aims of this study were to report two cases of highly PEN-R VGS IE, perform a literature review, and evaluate various antibiotic combinations in vitro and in vivo The following combinations were tested by time-kill studies and in the rabbit experimental endocarditis (EE) model: PEN-gentamicin, ceftriaxone-gentamicin, vancomycin-gentamicin, daptomycin-gentamicin, and daptomycin-ampicillin. Case 1 was caused by Streptococcus parasanguinis (PEN MIC, 4 μg/ml) and was treated with vancomycin plus cardiac surgery. Case 2 was caused by Streptococcus mitis (PEN MIC, 8 μg/ml) and was treated with 4 weeks of vancomycin plus gentamicin, followed by 2 weeks of vancomycin alone. Both patients were alive and relapse-free after ≥6 months follow-up. For the in vitro studies, except for daptomycin-ampicillin, all combinations demonstrated both synergy and bactericidal activity against the S. parasanguinis isolate. Only PEN-gentamicin, daptomycin-gentamicin, and daptomycin-ampicillin demonstrated both synergy and bactericidal activity against the S. mitis strain. Both strains developed high-level daptomycin resistance (HLDR) during daptomycin in vitro passage. In the EE studies, PEN alone failed to clear S. mitis from vegetations, while ceftriaxone and vancomycin were significantly more effective (P < 0.001). The combination of gentamicin with PEN or vancomycin increased bacterial eradication compared to that with the respective monotherapies. In summary, two patients with highly PEN-R VGS IE were cured using vancomycin-based therapy. In vivo, regimens of gentamicin plus either β-lactams or vancomycin were more active than their respective monotherapies. Further clinical studies are needed to confirm the role of vancomycin-based regimens for highly PEN-R VGS IE. The emergence of HLDR among these strains warrants caution in the use of daptomycin therapy for VGS IE.

Mutations in cdsA and pgsA Correlate with Daptomycin Resistance in Streptococcus mitis and S. oralis

We investigated the ability of several recent clinical viridans group streptococci (VGS) bloodstream isolates (Streptococcus mitis/S. oralis subgroup) from daptomycin (DAP)-naive patients to develop DAP resistance in vitro All strains rapidly developed high-level and stable DAP resistance. Substitutions in two enzymes involved in the cardiolipin biosynthesis pathway were identified, i.e., CdsA (phosphatidate cytidylyltransferase) and PgsA (CDP-diacylglycerol-glycerol-3-phosphate-3-phosphatidyltransferase). These mutations were associated with complete disappearance of phosphatidylglycerol and cardiolipin from cell membranes. DAP interactions with the cell membrane differed in isolates with PgsA versus CdsA substitutions.

Microbiological and clinical profile of infective endocarditis patients: an observational study experience from tertiary care center Karachi Pakistan

Background

The study analyzed microbiological and antimicrobial susceptibility profile of organisms isolated from patients with infective endocarditis (2015–17) and compared disease outcomes in cohorts of endocarditis patient with history of prior invasive vascular intervention (high risk group) vs those without (native valve group). We hypothesized that high risk group would be more likely to have severe disease outcomes.

Methods

This was a prospective cohort study (2015–17). All blood and cardiac tissue samples of enrolled patients suspected of endocarditis according to modified Duke’s criteria were followed for microbiological and antimicrobial susceptibility profile. The high risk group was compared with the native valve group with 90 day follow up to determine difference in clinical course and outcome in terms of disease severity (defined as any patient with endocarditis undergoing surgical management, readmission or dying). The data was analyzed using SPSS 21.0 software and chi-square test. 90 day mortality was calculated using Kaplan Meier survival curves.

Results

Total 104 patients with endocarditis were enrolled. Overall culture positivity rate was 71.2%. Streptococcus species were the most common isolate (36.7%), followed by S. aureus (17.3%) cases. In Streptococcus species, 14.2% showed intermediate susceptibility to penicillin. Thirty six patients were included in the cohort analysis. A poor outcome was seen in 85.7% high risk group as compared to 50% of native valve group. The overall mortality rate was 19.4%.

Conclusions

We found Streptococcus species to be the predominant pathogen for endocarditis overall. However Staphylococcus aureus predominated native valve group. High risk group showed more complicated clinical course.

Electronic supplementary material

The online version of this article (10.1186/s13019-018-0781-y) contains supplementary material, which is available to authorized users.

Prevention of High-Level Daptomycin-Resistance Emergence In Vitro in Streptococcus mitis-oralis by Using Combination Antimicrobial Strategies

Among the viridans group streptococci, S. mitis-oralis strains are frequently resistant to multiple β-lactams and tolerant to vancomycin (VAN). This scenario has led to the proposed clinical use of newer agents, like daptomycin (DAP) for such S. mitis-oralis strains. However, recent recognition of the rapid and durable emergence of high-level DAP-resistance (DAP-R; DAP MICs > 256 µg/ml) induced by DAP exposures in vitro and in vivo has dampened enthusiasm for such approaches. In this study, we evaluated a broad range of DAP combination regimens in vitro for their capacity to prevent emergence of high-level DAP-R in a prototype S. mitis-oralis strain (351) during serial passage experiments, including DAP + either gentamicin (GEN), rifampin (RIF), trimethoprim-sulfamethoxazole (TMP-SMX), imipenem (IMP), ceftaroline (CPT), tedizolid (TDZ), or linezolid (LDZ). In addition, we assessed selected DAP combination regimens for their ability to exert either an early bactericidal impact and/or synergistically kill the S. mitis-oralis study strain. During serial passage, three of the eight antibiotic combinations (DAP + GEN, CPT, or TMP- SMX) exhibited significantly reduced DAP MICs (≈ by 8-40 fold) vs serial exposure in DAP alone (DAP MICs > 256 µg/ml). In addition, combinations of DAP + GEN and DAP + CPT were both bactericidal and synergistic in early time-kill curve interactions.

Comparative in vitro activity of ceftaroline and comparator agents against nosocomial Gram-negative and Gram-positive clinically significant bacterial isolates from patients in a teaching hospital in Kuwait

The objective was to test the in vitro activities of ceftaroline and comparator agents against clinical isolates of Gram-negative and Gram-positive bacteria. Isolates were identified with VITEK II. Susceptibility testing was with E test. A total of 1264 isolates were tested. Compared to other cephalosporins, ceftaroline demonstrated excellent in vitro activities (MIC90, ≤0.5 mg/L) against Escherichia coli, Salmonella spp. and Haemophilus influenzae. When matched with the comparator cephalosporins, ceftaroline demonstrated the greatest activity against methicillin-susceptible Staphylococcus aureus (MSSA), with MIC90 of 0.25 mg/L. Ceftaroline's MIC90s against both community-associated methicillin-resistant S. aureus (MRSA) and hospital-acquired MRSA were 0.5 and 1 mg/L, respectively. Major discrepancies were noted between E test and disc diffusion tests for ceftaroline only against 16 Gram-negative and 16 Gram-positive isolates. Ceftaroline demonstrated an excellent in vitro activity against the majority of clinically significant Gram-negative and Gram-positive isolates obtained from proven cases of bacterial infections.

Evaluation of daptomycin combinations with cephalosporins or gentamicin against Streptococcus mitis group strains in an in vitro model of simulated endocardial vegetations (SEVs)

Objectives

Among viridans group streptococcal infective endocarditis (IE), the Streptococcus mitis group is the most common aetiological organism. Treatment of IE caused by the S. mitis group is challenging due to the high frequency of β-lactam resistance, drug allergy and intolerability of mainstay antimicrobial agents such as vancomycin or gentamicin. Daptomycin has been suggested as an alternative therapeutic option in these scenarios based on its excellent susceptibility profile against S. mitis group strains . However, the propensity of many S. mitis group strains to rapidly evolve stable, high-level daptomycin resistance potentially limits this approach.

Methods

We evaluated the activity of 6 mg/kg/day daptomycin alone or in combination with gentamicin, ceftriaxone or ceftaroline against two daptomycin-susceptible S. mitis group strains over 96 h in a pharmacokinetic/pharmacodynamic model of simulated endocardial vegetations.

Results

Daptomycin alone was not bactericidal and high-level daptomycin resistance evolved at 96 h in both organisms. Combinations of daptomycin + ceftriaxone and daptomycin + ceftaroline demonstrated enhanced killing activity compared with each antibiotic alone and prevented emergence of daptomycin resistance at 96 h. Use of gentamicin as an adjunctive agent neither improved the efficacy of daptomycin nor prevented the development of daptomycin resistance.

Conclusions

Addition of ceftriaxone or ceftaroline to daptomycin improves the bactericidal activity against S. mitis group strains and prevents daptomycin resistance emergence. Further investigation with combinations of daptomycin and β-lactams in a large number of strains is warranted to fully elucidate the clinical implications of such combinations for treatment of S. mitis group IE.

High proportions of Staphylococcus epidermidis in dental caries harbor multiple classes of antibiotics resistance, significantly increase inflammatory interleukins in dental pulps

Staphylococcus epidermidis is one of most prevalent in dental caries or dental pulp which has the capability of horizontal genetic transfer between different bacterial species in the oropharynx, suggesting that it may evolve with the dissemination of resistant determinants, This study was performed to molecularly characterize and differentiate S. epidermidis isolated from dental caries and healthy individual. Also, two important cytokines in inflammation were assayed caused due to S. epidermidis of health and dental caries sources. Dental caries strains were more resistant with high MIC ₅₀ and MIC ₉₀ value. These isolates also showed the presence of mecA gene and another virulence gene i. e sea and seb comparatively more than healthy individual isolates. SCCmec types, III and IV was more prevalent in dental caries isolates where an as healthy individual was more non-typable. Additionally, the quantity of IL-1β and IL-8 caused due to dental caries isolates was seen more which indicate dental caries isolates are able to induce. This study showed that S. epidermidis a normal flora of oropharyngeal are more diverse to those strains which cause dental caries. S. epidermidis owns a prodigious genetic plasticity that permits to obtain, lose or regulate genetic elements that provide compensations to improve its colonization in the host.

Impact of High-Level Daptomycin Resistance in the Streptococcus mitis Group on Virulence and Survivability during Daptomycin Treatment in Experimental Infective Endocarditis

Among the viridans group streptococci, the Streptococcus mitis group is the most common cause of infective endocarditis. These bacteria have a propensity to be β-lactam resistant, as well as to rapidly develop high-level and durable resistance to daptomycin (DAP). We compared a parental, daptomycin-susceptible (DAPs) S. mitis/S. oralis strain and its daptomycin-resistant (DAPr) variant in a model of experimental endocarditis in terms of (i) their relative fitness in multiple target organs in this model (vegetations, kidneys, spleen) when animals were challenged individually and in a coinfection strategy and (ii) their survivability during therapy with daptomycin-gentamicin (an in vitro combination synergistic against the parental strain). The DAPr variant was initially isolated from the cardiac vegetations of animals with experimental endocarditis caused by the parental DAPs strain following treatment with daptomycin. The parental strain and the DAPr variant were comparably virulent when animals were individually challenged. In contrast, in the coinfection model without daptomycin therapy, at both the 106- and 107-CFU/ml challenge inocula, the parental strain outcompeted the DAPr variant in all target organs, especially the kidneys and spleen. When the animals in the coinfection model of endocarditis were treated with DAP-gentamicin, the DAPs strain was completely eliminated, while the DAPr variant persisted in all target tissues. These data underscore that the acquisition of DAPr in S. mitis/S. oralis does come at an intrinsic fitness cost, although this resistance phenotype is completely protective against therapy with a potentially synergistic DAP regimen.

Perturbations of Phosphatidate Cytidylyltransferase (CdsA) Mediate Daptomycin Resistance in Streptococcus mitis/oralis by a Novel Mechanism

Streptococcus mitis/oralis is an important pathogen, causing life-threatening infections such as endocarditis and severe sepsis in immunocompromised patients. The β-lactam antibiotics are the usual therapy of choice for this organism, but their effectiveness is threatened by the frequent emergence of resistance. The lipopeptide daptomycin (DAP) has been suggested for therapy against such resistant S. mitis/oralis strains due to its in vitro bactericidal activity and demonstrated efficacy against other Gram-positive pathogens. Unlike other bacteria, however, S. mitis/oralis has the unique ability to rapidly develop stable, high-level resistance to DAP upon exposure to the drug both in vivo and in vitro Using isogenic DAP-susceptible and DAP-resistant S. mitis/oralis strain pairs, we describe a mechanism of resistance to both DAP and cationic antimicrobial peptides that involves loss-of-function mutations in cdsA (encoding a phosphatidate cytidylyltransferase). CdsA catalyzes the synthesis of cytidine diphosphate-diacylglycerol, an essential phospholipid intermediate for the production of membrane phosphatidylglycerol and cardiolipin. DAP-resistant S. mitis/oralis strains demonstrated a total disappearance of phosphatidylglycerol, cardiolipin, and anionic phospholipid microdomains from membranes. In addition, these strains exhibited cross-resistance to cationic antimicrobial peptides from human neutrophils (i.e., hNP-1). Interestingly, CdsA-mediated changes in phospholipid metabolism were associated with DAP hyperaccumulation in a small subset of the bacterial population, without any binding by the remaining larger population. Our results indicate that CdsA is the major mediator of high-level DAP resistance in S. mitis/oralis and suggest a novel mechanism of bacterial survival against attack by antimicrobial peptides of both innate and exogenous origins.

Antibiotic Resistance of Non-pneumococcal Streptococci and Its Clinical Impact

The taxonomy of streptococci has undergone major changes during the last two decades. The present classification is based on both phenotypic and genotypic data. Phylogenetic classification of streptococci is based on 16S rRNA sequences [1], and it forms the backbone of the overall classification system of streptococci. Phenotypic properties are also important, especially for clinical microbiologists. The type of hemolysis on blood agar, reaction with Lancefield grouping antisera, resistance to optochin, and bile solubility remain important for grouping of clinical Streptococcus isolates and therefore treatment options [2]. In the following chapter, two phenotypic classification groups, viridans group streptococci (VGS) and beta-hemolytic streptococci, will be discussed.

Antibiotic Resistance in Severe Orofacial Infections

PURPOSE

This study assessed the antibiotic resistance profile in patients with severe orofacial infections treated at a single institution from 2009 through 2014. Factors contributing to resistance were studied. The resistance profile was compared with that of a cohort of similar patients treated a decade previously to identify changes in antibiotic resistance. In addition, the effect of antibiotic resistance on in-hospital course was studied.

MATERIALS AND METHODS

This was a 5-year retrospective cohort study. Patients were identified through the oral and maxillofacial surgery data registry. Inclusion criteria were patients treated for orofacial infection requiring hospital admission, surgical drainage, and availability of complete medical, surgical, and microbiological data. Patients with incomplete data or treated as outpatients or nonsurgically were excluded. Sixty patient charts were identified for review. Demographic data; medical, dental, and surgical histories; and hospital course and treatment specifics were obtained for each patient. Linear regression and logistic analyses were used to analyze the data.

RESULTS

Men composed 60% of the cohort (mean age, 45 yr). Average hospital stay was 5.5 days. Penicillin resistance was found in 32.5% of aerobic isolates and clindamycin resistance was found in 29.3%. Streptococcus viridans and Staphylococcus species showed increased resistance to clindamycin and erythromycin compared with historic controls. Younger patient age, surgical history, and number of cultured aerobes showed a relevant correlation to antibiotic resistance. The need for changes in antibiotics, repeat surgical drainage, and increased serum urea nitrogen levels correlated with longer hospital stay.

CONCLUSION

A serious increase in clindamycin and erythromycin resistance was found for S viridans and Staphylococcus species. Age, surgical history, and number of cultured aerobes showed a statistically meaningful correlation to antibiotic resistance. Presence of antibiotic resistance failed to show statistically relevant correlations to prolongation of hospital stay. Rather, the need for change in antibiotic regimen, the need for re-drainage, and increased serum urea nitrogen level were associated with longer hospital stay.

Oral cavities of healthy infants harbour high proportions of Streptococcus salivarius strains with phenotypic and genotypic resistance to multiple classes of antibiotics

Emerging antibiotic resistance in the oropharyngeal microbiota, of which Streptococcus salivarius is a prominent species, represents a challenge for treating paediatric populations. In this study, we investigated the role of Streptococcussalivarius as a reservoir for antibiotic resistance genes (ARG) in the oral microbiota by analysing 95 Streptococcussalivarius isolates from 22 healthy infants (2-16 months of age). MICs of penicillin G, amoxicillin, erythromycin, tetracycline, doxycycline and streptomycin were determined. ARG profiles were assessed in a subset of 21 strains by next-generation sequencing of genomes, followed by searches of assembled reads against the Comprehensive Antibiotic Resistance Database. Strains resistant to erythromycin, penicillins and tetracyclines were isolated from 83.3, 33.3 and 16.6 %, respectively, of infants aged 2 to 8 months with no prior antibiotic treatment. These percentages were100.0, 66.6 and 50.0 %, by 13 to 16 months of age. ARG or polymorphisms associated with antibiotic resistance were the most prevalent and involved genes for macrolide efflux (mel, mefA/E and macB), ribosomal protection [erm(B), tet(M) and tet(O)] and β-lactamase-like proteins. Phylogenetically related strains showing multidrug-resistant phenotypes harboured multidrug efflux ARG. Polymorphic genes associated with antibiotic resistance to drugs affecting DNA replication, folate synthesis, RNA/protein synthesis and regulators of antibiotic stress responses were detected. These data imply that Streptococcussalivarius strains established during maturation of the oral microbiota harbour a diverse array of functional ARG, even in the absence of antibiotic selective pressures, highlighting a potential role for this species in shaping antibiotic susceptibility profiles of oropharyngeal communities.

Biological and Epidemiological Features of Antibiotic-Resistant Streptococcus pneumoniae in Pre- and Post-Conjugate Vaccine Eras: a United States Perspective

Streptococcus pneumoniae inflicts a huge disease burden as the leading cause of community-acquired pneumonia and meningitis. Soon after mainstream antibiotic usage, multiresistant pneumococcal clones emerged and disseminated worldwide. Resistant clones are generated through adaptation to antibiotic pressures imposed while naturally residing within the human upper respiratory tract. Here, a huge array of related commensal streptococcal strains transfers core genomic and accessory resistance determinants to the highly transformable pneumococcus. β-Lactam resistance is the hallmark of pneumococcal adaptability, requiring multiple independent recombination events that are traceable to nonpneumococcal origins and stably perpetuated in multiresistant clonal complexes. Pneumococcal strains with elevated MICs of β-lactams are most often resistant to additional antibiotics. Basic underlying mechanisms of most pneumococcal resistances have been identified, although new insights that increase our understanding are continually provided. Although all pneumococcal infections can be successfully treated with antibiotics, the available choices are limited for some strains. Invasive pneumococcal disease data compiled during 1998 to 2013 through the population-based Active Bacterial Core surveillance program (U.S. population base of 30,600,000) demonstrate that targeting prevalent capsular serotypes with conjugate vaccines (7-valent and 13-valent vaccines implemented in 2000 and 2010, respectively) is extremely effective in reducing resistant infections. Nonetheless, resistant non-vaccine-serotype clones continue to emerge and expand.

Impact of amoxicillin therapy on resistance selection in patients with community-acquired lower respiratory tract infections: a randomized, placebo-controlled study

OBJECTIVES

To determine the effect of amoxicillin treatment on resistance selection in patients with community-acquired lower respiratory tract infections in a randomized, placebo-controlled trial.

METHODS

Patients were prescribed amoxicillin 1 g, three times daily (n = 52) or placebo (n = 50) for 7 days. Oropharyngeal swabs obtained before, within 48 h post-treatment and at 28-35 days were assessed for proportions of amoxicillin-resistant (ARS; amoxicillin MIC ≥2 mg/L) and -non-susceptible (ANS; MIC ≥0.5 mg/L) streptococci. Alterations in amoxicillin MICs and in penicillin-binding-proteins were also investigated. ITT and PP analyses were conducted.

RESULTS

ARS and ANS proportions increased 11- and 2.5-fold, respectively, within 48 h post-amoxicillin treatment compared with placebo [ARS mean increase (MI) 9.46, 95% CI 5.57-13.35; ANS MI 39.87, 95% CI 30.96-48.78; P < 0.0001 for both]. However, these differences were no longer significant at days 28-35 (ARS MI -3.06, 95% CI -7.34 to 1.21; ANS MI 4.91, 95% CI -4.79 to 14.62; P > 0.1588). ARS/ANS were grouped by pbp mutations. Group 1 strains exhibited significantly lower amoxicillin resistance (mean MIC 2.8 mg/L, 95% CI 2.6-3.1) than group 2 (mean MIC 9.3 mg/L, 95% CI 8.1-10.5; P < 0.0001). Group 2 strains predominated immediately post-treatment (61.07%) and although decreased by days 28-35 (30.71%), proportions remained higher than baseline (18.70%; P = 0.0004).

CONCLUSIONS

By utilizing oropharyngeal streptococci as model organisms this study provides the first prospective, experimental evidence that resistance selection in patients receiving amoxicillin is modest and short-lived, probably due to 'fitness costs' engendered by high-level resistance-conferring mutations. This evidence further supports European guidelines that recommend amoxicillin when an antibiotic is indicated for community-acquired lower respiratory tract infections.

Zinc oxide and titanium dioxide nanoparticles induce oxidative stress, inhibit growth, and attenuate biofilm formation activity of Streptococcus mitis

Streptococcus mitis from the oral cavity causes endocarditis and other systemic infections. Rising resistance against traditional antibiotics amongst oral bacteria further aggravates the problem. Therefore, antimicrobial and antibiofilm activities of zinc oxide and titanium dioxide nanoparticles (NPs) synthesized and characterized during this study against S. mitis ATCC 6249 and Ora-20 were evaluated in search of alternative antimicrobial agents. ZnO and TiO2-NPs exhibited an average size of 35 and 13 nm, respectively. The IC50 values of ZnO and TiO2-NPs against S. mitis ATCC 6249 were 37 and 77 µg ml(-1), respectively, while the IC50 values against S. mitis Ora-20 isolate were 31 and 53 µg ml(-1), respectively. Live and dead staining, biofilm formation on the surface of polystyrene plates, and extracellular polysaccharide production show the same pattern. Exposure to these nanoparticles also shows an increase (26-83 %) in super oxide dismutase (SOD) activity. Three genes, namely bapA1, sodA, and gtfB like genes from these bacteria were identified and sequenced for quantitative real-time PCR analysis. An increase in sodA gene (1.4- to 2.4-folds) levels and a decrease in gtfB gene (0.5- to 0.9-folds) levels in both bacteria following exposure to ZnO and TiO2-NPs were observed. Results presented in this study verify that ZnO-NPs and TiO2-NPs can control the growth and biofilm formation activities of these strains at very low concentration and hence can be used as alternative antimicrobial agents for oral hygiene.

Antibiotic susceptibility and molecular mechanisms of macrolide resistance in streptococci isolated from adult cystic fibrosis patients

The cystic fibrosis (CF) airways are colonized by polymicrobial communities with high bacterial load and are influenced by frequent antibiotic exposures. This community includes diverse streptococci, some of which have been directly or indirectly associated with pulmonary exacerbations. As many streptococci are naturally competent, horizontal transfer of antibiotic-resistant determinants coupled with frequent and/or chronic antibiotic exposure may contribute to high resistance rates. In this study, we assessed antibiotic resistance in 413 streptococcal isolates from adult CF patients against nine antibiotics relevant in CF treatment. We observed very low rates of cephalosporin resistance [cefepime and ceftriaxone ( < 2%)], and higher rates of resistance to tetracycline (∼34%) and sulfamethoxazole/trimethoprim (∼45%). The highest rate of antibiotic resistance was to the macrolides [azithromycin (56.4%) and erythromycin (51.6%)]. We also investigated the molecular mechanisms of macrolide resistance and found that only half of our macrolide-resistant streptococci isolates contained the mef (efflux pump) or erm (methylation of 23S ribosomal target site) genes. The majority of isolates were, however, found to have point mutations at position 2058 or 2059 of the 23S ribosomal subunit - a molecular mechanism of resistance not commonly reported in the non-pyogenic and non-pneumococcal streptococci, and unique in comparison with previous studies. The high rates of resistance observed here may result in poor outcomes where specific streptococci are contributing to CF airway disease and serve as a reservoir of resistance genes within the CF airway microbiome.

Inhibition of Oral Streptococci Growth Induced by the Complementary Action of Berberine Chloride and Antibacterial Compounds

Synergistic interactions between natural bioactive compounds from medicinal plants and antibiotics may exhibit therapeutic benefits, acting against oral cariogenic and opportunistic pathogens. The aim of the presented work was to assess the antibacterial activity of berberine chloride (BECl) in light of the effect exerted by common antibiotics on selected reference strains of oral streptococci (OST), and to evaluate the magnitude of interactions. Three representative oral microorganisms were investigated: Streptococcus mutans ATCC 25175 (SM), S. sanguinis ATCC 10556 (SS), S. oralis ATCC 9811 (SO) and microdilution tests, along with disc diffusion assays were applied. Here, we report that growth (viability) of all oral streptococci was reduced by exposure to BECl and was dependent primarily on exposure/incubation time. A minimum inhibitory concentrations (MIC) of BECl against OST ranged from 512 µg/mL (SS) to 1024 µg/mL (SM, SO). The most noticeable antibacterial effects were observed for S. sanguinis (MIC 512 µg/mL) and the most significant synergistic action was found for the combinations BECl-penicillin, BECl-clindamycin and BECl-erythromycin. The S. oralis reflects the highest MBC value as assessed by the AlamarBlue assay (2058 µg/mL). The synergy between berberine and common antibiotics demonstrates its potential use as a novel antibacterial tool for opportunistic infections and also provides a rational basis for the use of berberine as an oral hygiene measure.

Commensal streptococci serve as a reservoir for β-lactam resistance genes in Streptococcus pneumoniae

Streptococcus pneumoniae is a leading cause of pneumonia, meningitis, septicemia, and middle ear infections. The incidence of S. pneumoniae isolates that are not susceptible to penicillin has risen worldwide and may be above 20% in some countries. Beta-lactam antibiotic resistance in pneumococci is associated with significant sequence polymorphism in penicillin-binding proteins (PBPs). Commensal streptococci, especially S. mitis and S. oralis, have been identified as putative donors of mutated gene fragments. However, no studies have compared sequences of the involved pbp genes in large collections of commensal streptococci with those of S. pneumoniae. We therefore investigated the sequence diversity of the transpeptidase region of the three pbp genes, pbp2x, pbp2b, and pbp1a in 107, 96, and 88 susceptible and nonsusceptible strains of commensal streptococci, respectively, at the nucleotide and amino acid levels to determine to what extent homologous recombination between commensal streptococci and S. pneumoniae plays a role in the development of beta-lactam resistance in S. pneumoniae. In contrast to pneumococci, extensive sequence variation in the transpeptidase region of pbp2x, pbp2b, and pbp1a was observed in both susceptible and nonsusceptible strains of commensal streptococci, conceivably reflecting the genetic diversity of the many evolutionary lineages of commensal streptococci combined with the recombination events occurring with intra- and interspecies homologues. Our data support the notion that resistance to beta-lactam antibiotics in pneumococci is due to sequences acquired from commensal Mitis group streptococci, especially S. mitis. However, several amino acid alterations previously linked to beta-lactam resistance in pneumococci appear to represent species signatures of the donor strain rather than being causal of resistance.

An Unusual Cause of Flexor Tenosynovitis: Streptococcus mitis

Summary:

Streptococcus mitis is a commensal organism of the human oropharynx that rarely causes infection in healthy individuals. Herein, we describe a previously healthy 35-year-old woman who presented with acute pyogenic flexor tenosynovitis of the left index finger due to S. mitis infection. The patient’s infection was treated successfully via surgical and medical interventions, and during follow-up, it was determined that she was complement component C3 deficient. Tenosynovitis is an emergent clinical syndrome that can result in permanent disability or amputation. To the best of our knowledge, this case report is the first to describe tenosynovitis due to S. mitis; in addition, it highlights the importance of initiating therapy with antibiotics that are effective against this rare pathogen.

Contemporary drug treatment of infective endocarditis

Infective endocarditis (IE) occurs at a rate of approximately 0.9-6.2 per 100,000 people per year and is associated with a high morbidity and mortality despite advancements in antibiotic and surgical treatments. The general approach to the treatment of IE is initial clinical stabilization, early acquisition of blood cultures, and definitive medical and/or surgical treatment. Surgical consultation should be obtained early when indicated in order to determine the best treatment approach for each individual patient. Surgery is indicated in most cases of prosthetic valve endocarditis, Staphylococcus aureus endocarditis, fungal endocarditis, and endocarditis associated with large vegetations (≥10 mm). Initial antibiotic therapy for IE should be targeted to the culprit microorganism; however, in some cases, empiric therapy must be initiated prior to definitive culture diagnosis. Empiric antibiotics should be targeted toward the most likely pathogens, including staphylococci, streptococci, and enterococci species. Here we discuss the recommended antibiotic regimens for the most common causes of IE as indicated by the American Heart Association and European Society of Cardiology. In 2008, the ACC/AHA published guideline updates on the treatment of valvular heart disease, which included a focused update on endocarditis prophylaxis. According to the most recent guidelines, the number of patients who require antibiotic prophylaxis has decreased substantially. Treatment of IE should be targeted toward the causative microorganism and must be based on the type and location of valve involved (native, prosthetic, left or right sided), the clinical status of the patient, and the likelihood for clinical success. This requires a collaborative effort from multiple medical specialties including infectious disease specialists, cardiologists, and cardiothoracic surgeons.

High Frequency of Isolation of Antibiotic-Resistant Oral Viridans Streptococci from Children in Kuwait

Antimicrobial resistance among the Viridans group of streptococci (VGS) has emerged as a hindrance to effective antibiotic therapy. Our objective was to evaluate the prevalence of antibiotic-resistant VGS in healthy children. Plaque samples were collected from tooth and tongue surfaces of 102 healthy subjects. Serially diluted samples were inoculated onto BHI agar plates and Mitis Salivarius Agar (MSA) plates and incubated as appropriate. Representative colonies were identified to species level by standard methods. Susceptibility of the VGS was performed by determining the minimum inhibitory concentrations (MICs) of 11 antibiotics using Etest. Of the 540 VGS isolates from both sites, 58% were from the tooth surfaces and 42% from the tongue. The most prevalent were S. salivarius (21.5%) and S. sanguis (16.3%). Imipenem and vancomycin had excellent activities. Resistance rates to trimethoprim, amoxicillin, piperacillin, erythromycin, cefuroxime and cephalothin, were 60.7, 40.8, 34.7, 32.6, 27.5 and 25.3%, respectively. Resistance rates to penicillin and clindamycin were 15.9% and 15.4%, respectively. The majority of the erythromycin-resistant isolates were from the tongue; 41% versus 29%. At the species level, 26% and 23% of S. salivarius and 23% and 14% of S. mutans from the tooth and tongue, respectively were resistant to penicillin. The data show species-related and site-related variations in the susceptibility pattern and an emerging high prevalence of antibiotic-resistant VGS. The difference in the susceptibilities between the species underscores the importance of accurate-identification and the need for surveillance of antimicrobial resistance among clinical isolates in our hospitals.

Infective endocarditis caused by multidrug-resistant Streptococcus mitis in a combined immunocompromised patient: an autopsy case report

An autopsy case of infective endocarditis caused by multidrug-resistant Streptococcus mitis was described in a patient with a combination of factors that compromised immune status, including autoimmune hemolytic anemia, post-splenectomy state, prolonged steroid treatment, and IgA deficiency. The isolated S. mitis strain from blood culture was broadly resistant to penicillin, cephalosporins, carbapenem, macrolides, and fluoroquinolone. Recurrent episodes of bacterial infections and therapeutic use of several antibiotics may underlie the development of multidrug resistance for S. mitis. Because clinically isolated S. mitis strains from chronically immunocompromised patients have become resistant to a wide spectrum of antibiotics, appropriate antibiotic regimens should be selected when treating invasive S. mitis infections in these compromised patients.

In vitro activity of cefepime against multidrug-resistant Gram-negative bacilli, viridans group streptococci and Streptococcus pneumoniae from a cross-Canada surveillance study

OBJECTIVE

To determine the in vitro activity of cefepime against multidrug-resistant Gram-negative bacilli and Gram-positive cocci obtained from an ongoing cross-Canada surveillance study.

DESIGN

Clinical isolates of aerobic Gram-negative bacilli with inducible and constitutive chromosomally mediated cephalosporinases, viridans group streptococci and Streptococcus pneumoniae were collected from laboratories serving hospitals, nursing homes and physician offices in the community from across Canada during 1996 and 1997. Laboratories were asked to submit only clinically relevant nonduplicate isolates for susceptibility testing. In vitro antimicrobial susceptibility testing was carried out on all isolates of Gram-negative and viridans group streptococci. S pneumoniae were characterized as penicillin susceptible, intermediately resistant or highly resistant. Nonsusceptible isolates were defined as being intermediately or highly resistant (minimal inhibitory concentrations [MIC] greater than 0.06 mg/L). Only isolates of S pneumoniae that were nonsusceptible to penicillin were selected for further study. MICs were determined using a microbroth dilution technique according to the National Committee of Clinical Laboratory Standards.

RESULTS

A total of 727 Gram-negative bacilli samples were collected. No resistance to cefepime was detected with Citrobacter freundii, Serratia marcescens, Morganella morganii and Enterobacter species. Of these strains, Enterobacter species and C freundii were the most resistant to ceftazidime, cefotaxime and ceftriaxone with MIC(90S) of 32 mg/L or greater and resistance rates of 6% or greater. Resistance rates of Pseudomonas aeruginosa and Acinetobacter species to cefepime were 4.8% and 3%, respectively. The two organisms had similar rates of resistance to ceftazidime. Less than 3% of the Gram-negative bacilli were resistant to imipenem and meropenem. There were 153 viridans group streptococci, of which 22 (14.4%) were resistant to penicillin. Of 1287 S pneumoniae samples, 193 (15%) were nonsusceptible to penicillin. Cefepime, ceftriaxone and cefotaxime had comparable activity against all isolates of viridans group streptococci and S pneumoniae.

CONCLUSIONS

Cefepime demonstrated excellent in vitro activity against Gram-negative bacilli with inducible and constitutive chromosomally mediated cephalosporinases, and had equal or superior activity versus comparator beta-lactams against all isolates of viridans group streptococci and S pneumoniae.

Molecular mechanisms of β-lactam resistance in Streptococcus pneumoniae

Alterations in the target enzymes for β-lactam antibiotics, the penicillin-binding proteins (PBPs), have been recognized as a major resistance mechanism in Streptococcus pneumoniae. Mutations in PBPs that confer a reduced affinity to β-lactams have been identified in laboratory mutants and clinical isolates, and document an astounding variability of sites involved in this phenotype. Whereas point mutations are selected in the laboratory, clinical isolates display a mosaic structure of the affected PBP genes, the result of interspecies gene transfer and recombination events. Depending on the selective β-lactam, different combinations of PBP genes and mutations within are involved in conferring resistance, and astoundingly in non-PBP genes as well.

Oral streptococcal bacteremia in hospitalized patients: taxonomic identification and clinical characterization

Oral streptococci have been associated with systemic diseases, including infective endocarditis and neutropenic bacteremia. We analyzed 58 recent oral streptococcal bloodstream isolates, and we obtained clinical and demographic data for source patients. The sodA gene was found to be a better target than the 16S-23S rRNA internal transcribed spacer for DNA sequence-based species identification. Together, Streptococcus mitis and Streptococcus oralis were significantly more likely than the 12 combined remaining species to be isolated from neutropenic patients.

Clinical and laboratory features of Streptococcus salivarius meningitis: a case report and literature review

Streptococcus salivarius is a normal member of the human oral microbiome that is an uncommon cause of invasive infections. Meningitis is a rare but increasingly reported infection caused by S. salivarius. Despite the growing number of reported cases, a comprehensive review of the literature on S. salivarius meningitis is lacking. We sought to gain a better understanding of the clinical presentation, evaluation, management, and outcome of S. salivarius meningitis by analyzing previously reported cases. In addition to a single case reported here, 64 previously published cases of meningitis were identified for this review. The collected data confirm that most patients presented with classical signs and symptoms of bacterial meningitis with a predominance of neutrophils in the cerebrospinal fluid (CSF) and hypoglycorrhachia. The majority of cases followed iatrogenic or traumatic CSF contamination. Most cases were diagnosed by CSF culture within one day of symptom onset. There was no clear evidence of predisposing co-morbid conditions in patients with meningitis, although in most case reports, limited information was given on the medical history of each patient. Outcomes were generally favorable with antibiotic management. Clinicians should suspect S. salivarius meningitis in patients presenting acutely after medical or surgical procedures involving the meninges.

Pathogenic alteration in severe burn wounds

The present study aims to define the trend of time related changes with local bacterial alteration of bacterial resistance in severe burns in our burn center during a 12-year period. Retrospective analysis of microbiological results on severely burned wounds between 1998 and 2009 was carried out. A study of 3615 microbial isolates was performed. Staphylococcus aureus was the most commonly isolated pathogen (38.2%) followed by A. baumannii (16.2%), Streptococcus viridans (11.4%), Pseudomonas aeruginosa (10.4%), coagulase-negative staphylococci (CNS, 9.2%). The species ratios of S. aureus and A. baumannii increased significantly from 1st to 8th week of hospitalization, while those of Streptococcus viridans, P. aeruginosa and coagulase-negative staphylococci decreased during the same period. Bacterial resistance rates were compared between the periods 1998-2003 and 2004-2009. Vancomycin remained as the most sensitive antibiotic in S. aureus including methicillin-resistant S. aureus (MRSA). It was very likely that the majority of infections caused by Streptococcus viridans, P. aeruginosa and coagulase-negative staphylococci occurred in the early stage of burn course and the majority of infections caused by A. baumannii occurred 4 weeks after admission. The use of different antibiotics was probably the major contributor to these trends.

Evaluation of the MicroScan MICroSTREP plus antimicrobial panel for testing β-hemolytic streptococci and viridans group streptococci

BACKGROUND

In order to determine the clinical usefulness of the MicroScan (Siemens Healthcare Diagnostics, USA) MICroSTREP plus antimicrobial panel (MICroSTREP) for testing antimicrobial susceptibility of β-hemolytic streptococci (BHS) and viridans group streptococci (VGS), we compared the accuracy of MICroSTREP with that of the CLSI reference method.

METHODS

Seventy-five BHS and 59 VGS isolates were tested for antimicrobial susceptibility to ampicillin, penicillin, cefotaxime, meropenem, erythromycin, clindamycin, levofloxacin, and vancomycin by using MICroSTREP and the CLSI agar dilution method.

RESULTS

The overall essential agreement with regard to minimum inhibitory concentrations (MICs) (within ±1 double dilution) between MICroSTREP and the CLSI reference method was 98.2%, and categorical agreement (CA) was 96.9%. For the BHS isolates, the CA for erythromycin was 96.0%, whereas that for cefotaxime, meropenem, levofloxacin, and vancomycin (for ampicillin, penicillin, and clindamycin; 98.7%) was 100%. For the VGS isolates, the CA for penicillin was 84.7% and that for erythromycin, clindamycin, and vancomycin (for meropenem, 86.5%; for ampicillin, 88.1%; and for cefotaxime and levofloxacin, 96.6%) was 100%. All categorical errors of penicillin and ampicillin in the VGS isolates were minor.

CONCLUSIONS

The accuracy of MICroSTREP is comparable to that of the CLSI reference method, suggesting that this panel can be effective for testing antimicrobial susceptibility of BHS and VGS.

Streptococcus mitis: walking the line between commensalism and pathogenesis

Streptococcus mitis is a viridans streptococcus and a normal commensal of the human oropharynx. However, S. mitis can escape from this niche and cause a variety of infectious complications including infective endocarditis, bacteraemia and septicaemia. It uses a variety of strategies to effectively colonize the human oropharynx. These include expression of adhesins, immunoglobulin A proteases and toxins, and modulation of the host immune system. These various colonization factors allow S. mitis to compete for space and nutrients in the face of its more pathogenic oropharyngeal microbial neighbours. However, it is likely that in vulnerable immune-compromised patients S. mitis will use the same colonization and immune modulation factors as virulence factors promoting its opportunistic pathogenesis. The recent publication of a complete genome sequence for S. mitis strain B6 will allow researchers to thoroughly investigate which genes are involved in S. mitis host colonization and pathogenesis. Moreover, it will help to give insight into where S. mitis fits in the complicated oral microbiome. This review will discuss the current knowledge of S. mitis factors involved in host colonization, their potential role in virulence and what needs to be done to fully understand how a an oral commensal successfully transitions to a virulent pathogen.

Microbially-produced peptides having potential application to the prevention of dental caries

Strategies advanced to decrease the occurrence of dental caries have in the past typically focussed upon attempting to reduce plaque accumulation by application of broad-spectrum antibacterial agents. In recent years however there has been growing interest in the application of a more targeted approach to the selective elimination from plaque of those bacterial species that are specifically implicated as the aetiological agents of this disease. This review focuses upon a number of the small bacterially-produced peptide antibiotics known as bacteriocins that are currently being explored for their potential role in the treatment and prevention of dental caries.

Macrolide and clindamycin resistance in Streptococcus milleri group isolates from the airways of cystic fibrosis patients

Organisms belonging to the Streptococcus milleri group (SMG) are known for their role in pyogenic infections but have recently been implicated as etiological agents of pulmonary exacerbation in adult patients with cystic fibrosis (CF). The prolonged exposure of CF patients to antibiotics prompted us to investigate the susceptibility profiles of 118 SMG isolates from the airways of CF patients to 12 antibiotics compared to 43 SMG isolates from patients with invasive infections. We found that approximately 60% of all isolates failed to grow using the standard medium for disc diffusion, Mueller-Hinton blood agar (MHBA), so we explored the usefulness of brain heart infusion (BHI) agar for susceptibility testing. Zone-of-inhibition comparisons between BHI and MHBA showed strong correlations for six antibiotics, and interpretations were similar for both medium types. For ceftriaxone and cefepime, both groups of isolates were highly susceptible. Tetracycline resistance levels were comparable between the two groups (22% in CF isolates and 17.4% in invasive isolates). However, more than half of the CF isolates were not susceptible to azithromycin, erythromycin, and clindamycin, compared to 11%, 13%, and 6.5% of invasive isolates, respectively. There were 5-fold and 8-fold increased risks of azithromycin and clindamycin resistance, respectively, for the isolates from the airways of CF patients relative to the invasive isolates. Macrolide resistance was strongly linked to chronic azithromycin therapy in CF patients. This study shows that BHI agar is a suitable alternative for antimicrobial susceptibility testing for the SMG and that SMG isolates from the airways of CF patients are more resistant to macrolides and clindamycin than strains isolated from patients with invasive infections.

Antimicrobial susceptibility of viridans group streptococcal blood isolates to eight antimicrobial agents

A total of 155 viridans group streptococci blood culture isolates identified by the Rapid ID32 Strep system were tested for their minimum inhibitory concentrations (MICs) to penicillin, amoxicillin, ceftriaxone, erythromycin, clindamycin, rifampicin, vancomycin and teicoplanin using the E-test. The following species were identified: S. oralis (n = 67), S. mitis (n = 66), S. sanguis (n = 7), S. salivarius (n = 5), S. parasanguis (n = 4), S. gordonii (n = 3) and S. mutans (n = 3). S. oralis and S. mitis demonstrated the highest levels of resistance to the agents tested. There were 27% of S. oralis isolates resistant to pencillin, 51% resistant to erythromycin and 6% resistant to clindamycin. For S. mitis 11% were resistant to penicillin, 40% resistant to erythromycin and 3% resistant to clindamycin. Penicillin resistant isolates (MIC > or = 2 mg/l) also demonstrated decreased susceptibility to other antimicrobial agents tested in this study. High level resistance (MIC > or = 2 mg/l) to ceftriaxone was found in 12 isolates. The isolates identified as ceftriaxone resistant comprised S. oralis (n = 7), S. mitis (n = 4) and S. parasanguis (n = 1). This study has highlighted the difference in susceptibility between different species of viridans group streptococci. These findings are of concern in the light of spread of antibiotic resistance genes from S. oralis and S. mitis to the more invasive pneumococcus.