Thymidine auxotrophic Staphylococcus aureus small-colony variant endocarditis and left ventricular assist device infection

Unravelling staphylococcal small-colony variants in cardiac implantable electronic device infections: clinical characteristics, management, and genomic insights

Objectives

Staphylococcal small-colony variants (SCVs) are common in cardiac implantable electronic device (CIED) infections. This is the first retrospective and multi-case study on CIED infections due to staphylococcal SCVs, aiming to provide a theoretical basis for the clinical management of CIED and device-related infections caused by staphylococcal SCVs.

Methods

Ninety patients with culture positive CIED infections were enrolled between 2021 and 2022. We compared the demographic and clinical characteristics of patients with and without SCVs and performed genomic studies on SCVs isolates.

Results

Compared to patients without SCVs, those with SCVs had a longer primary pacemaker implantation time and were more likely to have a history of device replacement and infection. They showed upregulated inflammatory indicators, especially higher NEUT% (52.6 vs. 26.8%, P = 0.032) and they had longer hospital stays (median 13 vs. 12 days, P = 0.012). Comparative genomics analysis was performed on Staphylococcus epidermidis wild-type and SCVs. Some genes were identified, including aap, genes encoding adhesin, CHAP domain-containing protein, LPXTG cell wall anchor domain-containing protein, and YSIRK-type signal peptide-containing protein.

Conclusion

Staphylococcal SCVs affect the clinical characteristics of CIED infections. The process of staphylococcal SCVs adherence, biofilm formation, and interaction with neutrophils play a vital role.

The transmutation of Escherichia coli ATCC 25922 to small colony variants (SCVs) E. coli strain as a result of exposure to gentamicin

BACKGROUND

Small colony variants (SCVs) are biotypes of bacteria that have a size of approximately one-tenth or less of the wild types and has distinct characteristics comparing to the wild type strains. Clinical SCVs are usually associated with persistent infection and require a long-term treatment program with antibiotics. In Saudi Arabia, there are few studies about SCVs Escherichia coli for this reason, this study is aimed to investigate the ability of gentamicin to mutate E. coli ATCC 25922 to produce small SCVs and investigate the genotypes and phenotypes changes and stress tolerance comparing to clinical SCVs E. coli and normal clinical E. coli Isolated from blood samples.

METHODS

In this investigation, four clinical blood samples were collected ted from patients and the cultivation and isolation were carried out in KFMC between December 2019 and February 2021. The identification of positive blood culture samples was done using phoenix MD. Non-SCV E. coli ATCC25922 were mutated to SCV using exposure to increasing gradual concentrations of gentamicin at 100-generation intervals. Biochemical features and susceptibility to standard antibiotics using automated Phoenix MD 50 and. The survival assays were done using several stresses including heat shock, low pH, high osmotic pressure, and oxidative pressure. Virulence genes screening included the detection of genes that encoded to α-haemolysin, CS12 fimbriae, F17-like fimbrial adhesion, P-related fimbriae, yersiniabactin siderophore system, P-fimbriae, aerobactin, iron-regulated genes using PCR and gel electrophoresis.

RESULTS

The data from the mutating E. coli ATCC 25922 small colony test with gentamicin revealed that the first emergence of the multidrug resistance (MDR) SCV E. coli strain occurred at generation number 250, corresponding to a gentamicin concentration of 57 g/ml. Pathogenicity islands detection revealed that all tested E. coli strains have PAI IV 536 genes on their chromosomes furthermore, mutated SCV E. coli ATCC 25922 acquired PAII CFT073 and PAI IV 536. Survival tests showed no significant differences changes in tolerance of mutated SCVs comparing to parental strain.

CONCLUSION

The present work concluded that gentamicin sub-MIC concentration gradual exposure can induce mutation responsible for SCV formation and evolving of MDR E. coli strains. The mutated SCVs evolved high-level aminoglycoside resistance for gentamicin and resistance to amikacin, it also developed resistance to 2 cephalosporin antibiotics cefuroxime, and cephalothin and a resistance to aztreonam.

Four decades of experience of prosthetic valve endocarditis reflect a high variety of diverse pathogens

Prosthetic valve endocarditis (PVE) remains a serious condition with a high mortality rate. Precise identification of the PVE-associated pathogen/s and their virulence is essential for successful therapy, and patient survival. The commonly described PVE-associated pathogens are staphylococci, streptococci and enterococci, with Staphylococcus aureus being the most frequently diagnosed species. Furthermore, multi-drug resistance pathogens are increasing in prevalence, and continue to pose new challenges mandating a personalized approach. Blood cultures in combination with echocardiography are the most common methods to diagnose PVE, often being the only indication, it exists. In many cases, the diagnostic strategy recommended in the clinical guidelines does not identify the precise microbial agent and to frequently, false negative blood cultures are reported. Despite the fact that blood culture findings are not always a good indicator of the actual PVE agent in the valve tissue, only a minority of re-operated prostheses are subjected to microbiological diagnostic evaluation. In this review, we focus on the diversity and the complete spectrum of PVE-associated bacterial, fungal and viral pathogens in blood, and prosthetic heart valve, their possible virulence potential, and their challenges in making a microbial diagnosis. We are curious to understand if the unacceptable high mortality of PVE is associated with the high number of negative microbial findings in connection with a possible PVE. Herein, we discuss the possibilities and limits of the diagnostic methods conventionally used and make recommendations for enhanced pathogen identification. We also show possible virulence factors of the most common PVE-associated pathogens and their clinical effects. Based on blood culture, molecular biological diagnostics, and specific valve examination, better derivations for the antibiotic therapy as well as possible preventive intervention can be established in the future.

Characteristics of Staphylococcus aureus small colony variants isolated from wound specimen of a tertiary care hospital in China

BACKGROUND

Small colony variants (SCVs) of Staphylococcus aureus (S. aureus) frequently lead to chronic and recurrent infections, but they are always ignored and there are few researches on their clinical isolates. We intended to investigate the prevalence and characteristics of S. aureus SCVs.

METHODS

None-duplicated S. aureus strains isolated from wound samples were collected from January 2018 to December 2020. The characteristics (i.e. colony morphology, growth rate, coagulase, biofilm formation, and pathogenic characteristics), antimicrobial susceptibilities, and resistance mechanisms of SCVs were also investigated. The genetic background of SCVs was analyzed through staphylococcal protein A (SPA) typing, sequence typing, and pulse field gel electrophoresis (PFGE).

RESULTS

Three SCVs were screened from 278 S. aureus strains (1.1%). They formed pinpoint white colonies on blood agar plates with weak hemolysis. The reproduction speed in liquid medium was very slow for SCVs strains. The coagulase weakened or disappeared, and the ability to form biofilm varied greatly. Only slight inflammation was triggered when wound infected. The SPA typing was t2592, t233, and t023, and the sequence typing was ST88, ST239, and ST965, respectively. The PFGE revealed three SCVs were singletons.

CONCLUSIONS

The rate of SCVs in wound sample is low in our hospital, and the formation is associated with the usage of antimicrobial. SCVs grow slowly, and their colony morphology and biochemical characteristics are significantly different from classic S. aureus. SCVs may cause chronic infection and weak inflammation. SCVs form in resistant or susceptible strains, and there is no clonal epidemic in this hospital.

Phenotypic Detection of Hemin-Inducible Trimethoprim-Sulfamethoxazole Heteroresistance in Staphylococcus aureus

Trimethoprim-sulfamethoxazole (SXT) is a valuable second-line antimicrobial agent to treat methicillin-resistant Staphylococcus aureus infections. Discrepancies between various antibiotic susceptibility testing (AST) methods for SXT susceptibility in S. aureus have been described. Here, we describe a hemin-inducible heteroresistance phenotype in S. aureus. We compared the results of the Vitek 2 AST on a set of 95 S. aureus clinical isolates with broth microdilution, disk diffusion using standard Mueller-Hinton agar, and disk diffusion using Mueller-Hinton agar supplemented with 5% horse blood (MHF). To investigate the potential clinical relevance of SXT heteroresistance, an in vivo Galleria mellonella infection assay was performed. All Vitek 2 SXT-susceptible (n = 17) isolates were concordant with AST results by other methods applied in this study. In 32/78 (41%) of Vitek 2 SXT-resistant isolates, we observed a heteroresistant growth phenotype on MHF. The heteroresistance phenotype was associated with the presence of dfr genes, encoding trimethoprim resistance. The addition of a hemin-impregnated disk in a double disk diffusion method on standard Mueller-Hinton agar was able to induce growth in the SXT zone of inhibition. An in vivo infection assay with G. mellonella suggested that the SXT heteroresistance phenotype resulted in lethality similar to that of the SXT-resistant phenotype. In this study, we describe a novel hemin-inducible heteroresistance phenotype in S. aureus. This heteroresistance phenotype may be missed by standard AST methods but can be detected by performing disk diffusion using Mueller-Hinton agar supplemented with 5% horse blood, commonly used for AST of fastidious organisms. This phenomenon may partly explain the discrepancies of AST methods in determining SXT resistance in S. aureus. IMPORTANCE Staphylococcus aureus is one of most important pathogens in clinical medicine. Besides its virulence, the acquisition or emergence of resistance toward antibiotic agents, in particular to beta-lactam antibiotics (methicillin-resistant S. aureus [MRSA]), poses a major therapeutic challenge. Trimethoprim-sulfamethoxazole (SXT) is one of the effective antimicrobial agents of last resort to treat MRSA infections. Here, we report the detection of a SXT-heteroresistant phenotype which is inducible by hemin and can be detected using Mueller-Hinton agar supplemented with horse blood. Heteroresistance describes the presence or emergence of resistant subpopulations, which may potentially lead to inaccurate antibiotic susceptibility testing results and influence the success of antibiotic therapy.

Emergence of Thymidine-Dependent Staphylococcus aureus Small-Colony Variants in Cystic Fibrosis Patients in Southern Brazil

We characterized Staphylococcus aureus small-colony variant (SCV) strains isolated from cystic fibrosis (CF) patients in southern Brazil. Smaller colonies of S. aureus were isolated from respiratory samples collected consecutively from 225 CF patients from July 2013 to November 2016. Two phenotypic methods—the auxotrophic classification and a modified method of antimicrobial susceptibility testing—were employed. PCR was conducted to detect the mecA, ermA, ermB, ermC, msrA, and msrB resistance genes. Furthermore, DNA sequencing was performed to determine the mutations in the thyA gene, and multilocus sequence typing was used to identify the genetic relatedness. S. aureus strains were isolated from 186 patients (82%); suggestive colonies of SCVs were obtained in 16 patients (8.6%). The clones CC1 (ST1, ST188, and ST2383), CC5 (ST5 and ST221), and ST398 were identified. Among SCVs, antimicrobial susceptibility testing showed that 77.7% of the isolates were resistant to multiple drugs, and all of them were susceptible to vancomycin. mecA (2), ermA (1), ermB (1), ermC (3), and msrB (18) were distributed among the isolates. Phenotypically thymidine-dependent isolates had different mutations in the thyA gene, and frameshift mutations were frequently observed. Of note, revertants showed nonconservative or conservative missense mutations. SCVs are rarely identified in routine laboratory tests.

IMPORTANCE Similar findings have not yet been reported in Brazil, emphasizing the importance of monitoring small-colony variants (SCVs). Altogether, our results highlight the need to improve detection methods and review antimicrobial therapy protocols in cystic fibrosis (CF) patients.

Distinct clonal lineages and within-host diversification shape invasive Staphylococcus epidermidis populations

S. epidermidis is a substantial component of the human skin microbiota, but also one of the major causes of nosocomial infection in the context of implanted medical devices. We here aimed to advance the understanding of S. epidermidis genotypes and phenotypes conducive to infection establishment. Furthermore, we investigate the adaptation of individual clonal lines to the infection lifestyle based on the detailed analysis of individual S. epidermidis populations of 23 patients suffering from prosthetic joint infection. Analysis of invasive and colonizing S. epidermidis provided evidence that invasive S. epidermidis are characterized by infection-supporting phenotypes (e.g. increased biofilm formation, growth in nutrient poor media and antibiotic resistance), as well as specific genetic traits. The discriminating gene loci were almost exclusively assigned to the mobilome. Here, in addition to IS256 and SCCmec, chromosomally integrated phages was identified for the first time. These phenotypic and genotypic features were more likely present in isolates belonging to sequence type (ST) 2. By comparing seven patient-matched nasal and invasive S. epidermidis isolates belonging to identical genetic lineages, infection-associated phenotypic and genotypic changes were documented. Besides increased biofilm production, the invasive isolates were characterized by better growth in nutrient-poor media and reduced hemolysis. By examining several colonies grown in parallel from each infection, evidence for genetic within-host population heterogeneity was obtained. Importantly, subpopulations carrying IS insertions in agrC, mutations in the acetate kinase (AckA) and deletions in the SCCmec element emerged in several infections. In summary, these results shed light on the multifactorial processes of infection adaptation and demonstrate how S. epidermidis is able to flexibly repurpose and edit factors important for colonization to facilitate survival in hostile infection environments.

S. epidermidis is a substantial component of the human skin microbiota, but also a major cause of nosocomial infections related to implanted medical devices. While phenotypic and genotypic determinants supporting invasion were identified, none appears to be necessary. By analysis of S. epidermidis from prosthetic joint infections, we here show that adaptive events are of importance during the transition from commensalism to infection. Adaptation to the infectious lifestyle is characterised by the development of intra-clonal heterogeneity, increased biofilm formation and enhanced growth in iron-free and nutrient-poor media, as well as reduced production of hemolysins. Importantly, during infection subpopulations emerge that carry mutations in a number of genes, most importantly the acetate kinase (ackA) and the β-subunit of the RNA polymerase (rpoB), have deleted larger chromosomal fragments (e.g. within the SCCmec element) or IS insertions in AgrC, a component of the master quorum sensing system in S. epidermidis. These results shed light on the multifactorial processes of infection adaptation and demonstrate how S. epidermidis is able to flexibly repurpose and edit factors important for colonization to facilitate survival under hostile infection conditions. While mobilome associated factors are important for S. epidermidis invasive potential, the species possesses a multi-layered and complex ability for adaptation to hostile environments, supporting the progression to chronic implant-associated infections.

Novel Research Models for Staphylococcus aureus Small Colony Variants (SCV) Development: Co-pathogenesis and Growth Rate

Staphylococcus aureus remains a great burden on the healthcare system. Despite prescribed treatments often seemingly to be successful, S. aureus can survive and cause a relapsing infection which cannot be cleared. These infections are in part due to quasi-dormant sub-population which is tolerant to antibiotics and able to evade the host immune response. These include Small Colony Variants (SCVs). Because SCVs readily revert to non-SCV cell types under laboratory conditions, the characterization of SCVs has been problematic. This mini-review covers the phenotypic and genetic changes in stable SCVs including the selection of SCVs by and interactions with other bacterial species.

Denture stomatitis associated with small-colony variants of Staphylococcus aureus: a case report

Background

The small-colony variants (SCVs) of Staphylococcus aureus were isolated from persistent and recurrent infections, especially after placement of medical devices having direct contact with human tissues. The emergence of SCVs is a survival strategy of S. aureus which enables them to hide inside host’s cells and induces a less severe immune response than to wild-type S. aureus. However, contrary to other medical devices, dental prosthesis as a surface potentially colonized by SCVs of S. aureus has not been examined thus far. We reported the first case of SCVs - S. aureus infection in denture wearer.

Case presentation

A 62-year-old woman with a complete removable acrylic denture presented extensive elevated erythematous lesions on the palate, compatible with denture stomatitis. The patient had a history of arterial hypertension, cigarette smoking and wearing denture at night. The fungal colonies, identified as Candida albicans, were cultured on Sabouraud agar. From three swabs (from hard palate mucosa, denture surface and angular cheilitis lesions) were cultured of pinpoint, clear, non-pigmented, and non-haemolytic colonies on Columbia agar. The small colonies turned out to be Gram-positive cocci, catalase-, Pastorex Staph Plus -, and clumping factor-positive, and oxidase-negative. Suspected phenotypically SCVs forms were definitively identified as S. aureus based on PCR amplification of species specific nuc and coa genes. Methicillin-resistance was verified by mecA gene detection. The isolates turned out to be susceptible to methicillin (MSSA) and resistant to gentamicin. The isolate was identified as menadione-auxotrophic variant.

Conclusions

This case demonstrated that oral cavity in denture wearers may be a reservoir of small-colony variants of S. aureus, besides C. albicans. The prevalence of these bacteria and their role in the pathogenesis of oral diseases are not understood. Due to problems with their detection and identification, the true prevalence of oral SCVs may be underestimated.

Phenotypic Stability of Staphylococcus Aureus Small Colony Variants (SCV) Isolates from Cystic Fibrosis (CF) Patients

One of the most interesting features of Staphylococcus aureus is its ability to switch to a small colony variant (SCV). This switch allows the pathogen to survive periods of antibiotic treatment or pressure from the immune system of the host and further enables it to start the infection once again after the environmental stress declines. However, so far only little is known about this reversion back to the more virulent wild type phenotype. Therefore, this study aimed to analyze the frequency of reversion to the wild type phenotype of thymidine auxotroph S. aureus SCV isolates (TD-SCVs) obtained from patients with cystic fibrosis (CF). With the use of single cell starting cultures, the occurrence of the thymidine prototroph revertants was monitored. The underlying mutational cause of the SCVs and subsequent revertants were analyzed by sequencing the gene coding for thymidylate synthase (ThyA), whose mutations are known to produce thymidine auxotroph S. aureus SCV. In our study, the underlying mutational cause for the switch to the TD-SCV phenotype was primarily point mutations. Out of twelve isolates, seven isolates showed an occurrence of revertants with a frequency ranging from 90.06% to 0.16%. This high variability in the frequency of reversion to the wild type was not expected. However, this variability in the frequency of reversion may also be the key to successful re-infection of the host. Sometimes quick reversion to the wild type proves necessary for survival, whereas other times, staying hidden for a bit longer leads to success in re-colonization of the host.

Staphylococcus aureus bloodstream infection in patients with ventricular assist devices-Management and outcome in a prospective bicenter cohort

OBJECTIVES

Ventricular assist devices (VAD) are increasingly implanted in patients with terminal heart failure. Here we describe the clinical course, management and outcome of VAD patients with S. aureus bloodstream infection (SAB).

METHODS

We conducted a post hoc analysis of data from 1073 patients who had been prospectively enrolled in two consecutive SAB bicenter cohort studies. Patients with VAD in situ at the onset of SAB were identified. Follow-up of patients was at least 90 days.

RESULTS

Twelve VAD patients with SAB were identified. Compared to the overall cohort, patients with VAD presented more often with fever (92% vs. 65%) and septic shock (33% vs. 23%) and showed higher C-reactive protein levels (mean 244 vs. 132 g/ml). The median time to onset of SAB after device implantation was 161 days (range 24-790 days). 30-day mortality was comparable to the whole cohort (17% vs. 19%). Infection-related surgical interventions were performed in six patients. Hematogenous dissemination to distant foci was not found in any patient. One out of nine surviving patients required continuous suppressive antibiotic therapy.

CONCLUSIONS

Mortality rates for VAD patients with SAB were comparable to SAB without VAD. No hematogenous disssemination or persistent infections were recorded, which might be associated with the prompt and aggressive antibiotic and surgical management in VAD patients. SAB per se does not preclude successful transplantation.

Genotypic variations between wild-type and small colony variant of Staphylococcus aureus in prosthetic valve infectious endocarditis: a comparative genomic and transcriptomic analysis

Staphylococcus aureus small colony variants (SCVs) can cause persistent infections. However, the genomes and transcriptomes of S. aureus SCVs remain poorly understood. A pair of isogenic wild-type and SCV methicillin-resistant S. aureus (MRSA) strains (IE1 and IE2, respectively) were isolated from a patient with prosthetic valve infectious endocarditis. The SCV strain IE2 grew more slowly than the wild-type strain, and serum killing and mouse lethality assays revealed that the virulence of SCV strain IE2 was decreased. Whole-genome sequencing of the SCV and wild-type strains revealed 15 mutations in nine genes associated with metabolism, virulence and DNA repair, including serine/threonine-protein kinase PrkC (prkC), glycerol-3-phosphate acyltransferase (plsY), 2-deoxyribose-5-phosphate aldolase (deoC), extracellular adherence protein (eap), iron compound ABC uptake transporter substrate-binding protein (sstD), RecU Holliday junction resolvase (recU), excinuclease ABC subunit B (uvrB), type I restriction-modification system, M subunit (hsdM) and smooth muscle caldesmon. Sequencing of RNA transcripts revealed that expression levels of 321 genes were upregulated and 582 genes were downregulated in SCV strain IE2 compared with IE1. Most of the differentially expressed genes were involved in metabolism. Expression levels of several genes involved in the pathways to which plsY, deoC, eap and sstD belonged were changed, associated with the metabolism and virulence of S. aureus. In conclusion, the reduced growth rate and decreased virulence of MRSA SCV strains may be related to mutations in and downregulation of genes associated with metabolism and virulence, especially plsY, deoC, eap and sstD.

Characterization of clinically isolated thymidine-dependent small-colony variants of Escherichia coli producing extended-spectrum β-lactamase

PURPOSE

Thymidine-dependent small-colony variants (TD-SCVs) are difficult to detect or test for antimicrobial susceptibility. We investigated the characteristics of clonal TD-SCVs of Escherichia coli, both with and without blaCTX-M-3, isolated from a patient.

METHODOLOGY

Mutation in the thyA gene was analysed by sequencing, and morphological abnormalities in the colonies and cells of the isolates were examined. Additionally, conjugational transfer experiments were performed to prove the horizontal transferability of plasmids harbouring resistance genes.

RESULTS

The TD-SCVs contained a single nucleotide substitution in the thyA gene, c.62G>A, corresponding to p.Arg21His. Morphologically, their colonies were more translucent and flattened than those of the wild-type strain. In addition, cells of the TD-SCVs were swollen and elongated, sometimes with abnormal and incomplete divisions; a large amount of cell debris was also observed. Changing c.62G>A back to the wild-type sequence reversed these abnormalities. Conjugational transfer experiments showed that the TD-SCV of E. coli with blaCTX-M-3 failed to transfer blaCTX-M-3 to E. coli CSH2. However, the TD-SCV of E. coli without blaCTX-M-3 experimentally received the plasmid encoding blaSHV-18 from Klebsiella pneumoniae ATCC 700603 and transferred it to E. coli CSH2.

CONCLUSION

Mutation in the thyA gene causes morphological abnormalities in the colonies and cells of E. coli, as well as inducing thymidine auxotrophy. In addition, TD-SCVs horizontally transmit plasmids encoding resistance genes. It is important to detect TD-SCVs based on their characteristics because they serve as reservoirs of transferable antibiotic resistance plasmids.

Antibiotic tolerance and the alternative lifestyles of Staphylococcus aureus

Staphylococcus aureus has an incredible ability to survive, either by adapting to environmental conditions or defending against exogenous stress. Although there are certainly important genetic traits, in part this ability is provided by the breadth of modes of growth S. aureus can adopt. It has been proposed that while within their host, S. aureus survives host-generated and therapeutic antimicrobial stress via alternative lifestyles: a persister sub-population, through biofilm growth on host tissue or by growing as small colony variants (SCVs). Key to an understanding of chronic and relapsing S. aureus infections is determining the molecular basis for its switch to these quasi-dormant lifestyles. In a multicellular biofilm, the metabolically quiescent bacterial community additionally produces a highly protective extracellular polymeric substance (EPS). Furthermore, there are bacteria within a biofilm community that have an altered physiology potentially equivalent to persister cells. Recent studies have directly linked the cellular ATP production by persister cells as their key feature and the basis for their tolerance of a range of antibiotics. In clinical settings, SCVs of S. aureus have been observed for many years; when cultured, these cells form non-pigmented colonies and are approximately ten times smaller than their counterparts. Various genotypic factors have been identified in attempts to characterize S. aureus SCVs and different environmental stresses have been implicated as important inducers.

Reduced Innate Immune Response to a Staphylococcus aureus Small Colony Variant Compared to Its Wild-Type Parent Strain

Background:Staphylococcus aureus (S. aureus) small colony variants (SCVs) can survive within the host intracellular milieu and are associated with chronic relapsing infections. However, it is unknown whether host invasion rates and immune responses differ between SCVs and their wild-type counterparts. This study used a stable S. aureus SCV (WCH-SK2^SCV) developed from a clinical isolate (WCH-SK2^WT) in inflammation-relevant conditions. Intracellular infection rates as well as host immune responses to WCH-SK2^WT and WCH-SK2^SCV infections were investigated. Method: NuLi-1 cells were infected with either WCH-SK2^WT or WCH-SK2^SCV, and the intracellular infection rate was determined over time. mRNA expression of cells infected with each strain intra- and extra-cellularly was analyzed using a microfluidic qPCR array to generate an expression profile of thirty-nine genes involved in the host immune response. Results: No difference was found in the intracellular infection rate between WCH-SK2^WT and WCH-SK2^SCV. Whereas, extracellular infection induced a robust pro-inflammatory response, intracellular infection elicited a modest response. Intracellular WCH-SK2^WT infection induced mRNA expression of TLR2, pro-inflammatory cytokines (IL1B, IL6, and IL12) and tissue remodeling factors (MMP9). In contrast, intracellular WCH-SK2^SCV infection induced up regulation of only TLR2. Conclusions: Whereas, host intracellular infection rates of WCH-SK2^SCV and WCH-SK2^WT were similar, WCH-SK2^SCV intracellular infection induced a less widespread up regulation of pro-inflammatory and tissue remodeling factors in comparison to intracellular WCH-SK2^WT infection. These findings support the current view that SCVs are able to evade host immune detection to allow their own survival.

Clinical Significance and Pathogenesis of Staphylococcal Small Colony Variants in Persistent Infections

Small colony variants (SCVs) were first described more than 100 years ago for Staphylococcus aureus and various coagulase-negative staphylococci. Two decades ago, an association between chronic staphylococcal infections and the presence of SCVs was observed. Since then, many clinical studies and observations have been published which tie recurrent, persistent staphylococcal infections, including device-associated infections, bone and tissue infections, and airway infections of cystic fibrosis patients, to this special phenotype. By their intracellular lifestyle, SCVs exhibit so-called phenotypic (or functional) resistance beyond the classical resistance mechanisms, and they can often be retrieved from therapy-refractory courses of infection. In this review, the various clinical infections where SCVs can be expected and isolated, diagnostic procedures for optimized species confirmation, and the pathogenesis of SCVs, including defined underlying molecular mechanisms and the phenotype switch phenomenon, are presented. Moreover, relevant animal models and suggested treatment regimens, as well as the requirements for future research areas, are highlighted.

Small-Colony Variants in Persistent and Recurrent Staphylococcus aureus Bacteremia

The small-colony variant (SCV) phenotype of Staphylococcus aureus is associated with intracellular persistence and reduced antimicrobial susceptibility, which can lead to therapeutic failure. Since SCVs grow slowly and have a confusing morphology, the identification of infections due to SCV is difficult. We have identified SCVs in two patients who presented with persistent methicillin-resistant Staphylococcus aureus (MRSA) bacteremia complicated by surgical site infections after cardiothoracic surgery. Nine blood isolates were collected from the two patients for species identification, antimicrobial susceptibility testing, and phenotypic and genotypic characterization. Colonies on Columbia blood agar were pinpoint, nonpigmented, nonhemolytic, and reverted to normal colonies after 48 hr of incubation on Schaedler agar. Auxotrophy assays revealed hemin dependence. Susceptibility to vancomycin (minimal inhibitory concentrations 1.0 μg/mL) was confirmed by E-test and broth microdilution test. All the isolates were identified as MRSA by multiplex polymerase chain reaction specific for the mecA, femA, and 16S rRNA genes, and all had the same genotype: Multilocus sequence typing ST5, SCCmec type II, agr type II, and spa type t2460. Moreover pulsed-field gel electrophoresis typing revealed that all nine isolates belonged to the same clone. Mutations in the relA gene were not found, and none of the isolates was identified as hVISA by population analysis profiling-AUC ratio. A high level of suspicion is required to detect SCVs, and although it is not common, the possibility of the SCV phenotype has to be considered in persistent S. aureus bacteremia.

Optimized In Vitro Antibiotic Susceptibility Testing Method for Small-Colony Variant Staphylococcus aureus

Staphylococcus aureus small-colony variants (SCVs) emerge frequently during chronic infections and are often associated with worse disease outcomes. There are no standardized methods for SCV antibiotic susceptibility testing (AST) due to poor growth and reversion to normal-colony (NC) phenotypes on standard media. We sought to identify reproducible methods for AST of S. aureus SCVs and to determine whether SCV susceptibilities can be predicted on the basis of treatment history, SCV biochemical type (auxotrophy), or the susceptibilities of isogenic NC coisolates. We tested the growth and stability of SCV isolates on 11 agar media, selecting for AST 2 media that yielded optimal SCV growth and the lowest rates of reversion to NC phenotypes. We then performed disk diffusion AST on 86 S. aureus SCVs and 28 isogenic NCs and Etest for a subset of 26 SCVs and 24 isogenic NCs. Growth and reversion were optimal on brain heart infusion agar and Mueller-Hinton agar supplemented with compounds for which most clinical SCVs are auxotrophic: hemin, menadione, and thymidine. SCVs were typically nonsusceptible to either trimethoprim-sulfamethoxazole or aminoglycosides, in accordance with the auxotrophy type. In contrast, SCVs were variably nonsusceptible to fluoroquinolones, macrolides, lincosamides, fusidic acid, and rifampin; mecA-positive SCVs were invariably resistant to cefoxitin. All isolates (both SCVs and NCs) were susceptible to quinupristin-dalfopristin, vancomycin, minocycline, linezolid, chloramphenicol, and tigecycline. Analysis of SCV auxotrophy type, isogenic NC antibiograms, and antibiotic treatment history had limited utility in predicting SCV susceptibilities. With clinical correlation, this AST method and these results may prove useful in directing treatment for SCV infections.

Phenotypic and Genotypic Characteristics of Small Colony Variants and Their Role in Chronic Infection

Small colony variant (SCV) bacteria arise spontaneously within apparently homogeneous microbial populations, largely in response to environmental stresses, such as antimicrobial treatment. They display unique phenotypic characteristics conferred in part by heritable genetic changes. Characteristically slow growing, SCVs comprise a minor proportion of the population from which they arise but persist by virtue of their inherent resilience and host adaptability. Consequently, SCVs are problematic in chronic infection, where antimicrobial treatment is administered during the acute phase of infection but fails to eradicate SCVs, which remain within the host causing recurrent or chronic infection. This review discusses some of the phenotypic and genotypic changes that enable SCVs to successfully proliferate within the host environment as potential pathogens and strategies that could ameliorate the resolution of infection where SCVs are present.

Evolution of Staphylococcus aureus under vancomycin selective pressure: the role of the small-colony variant phenotype

Staphylococcus aureus small-colony variants (SCVs) often persist despite antibiotic therapy. Against a 10(8)-CFU/ml methicillin-resistant S. aureus (MRSA) (strain COL) population of which 0%, 1%, 10%, 50%, or 100% was an isogenic hemB knockout (Ia48) subpopulation displaying the SCV phenotype, vancomycin achieved maximal reductions of 4.99, 5.39, 4.50, 3.28, and 1.66 log10 CFU/ml over 48 h. Vancomycin at ≥16 mg/liter shifted a population from 50% SCV cells at 0 h to 100% SCV cells at 48 h, which was well characterized by a Hill-type model (R2>0.90).

The novel species Streptococcus tigurinus and its association with oral infection

Streptococcus tigurinus is a novel species of viridans streptococci, shown to cause severe invasive infections such as infective endocarditis, spondylodiscitis and meningitis. S. tigurinus belongs to the Streptococcus mitis group and is most closely related to Streptococcus mitis, Streptococcus oralis, Streptococcus pneumoniae, Streptococcus pseudopneumoniae and Streptococcus infantis. The presence of S. tigurinus in the human oral cavity has been documented, including in patients with periodontal disease. This review addresses the available scientific knowledge on S. tigurinus and its association with closely related streptococci, and discusses its putative involvement in common oral infections. While there is as yet no strong evidence on the involvement of S. tigurinus with oral infections, its presence in the oral cavity and its association with endocarditis warrants special attention for a link between oral and systemic infection.

Characterization of Streptococcus tigurinus small-colony variants causing prosthetic joint infection by comparative whole-genome analyses

Small-colony variants (SCVs) of bacteria are associated with recurrent and persistent infections. We describe for the first time SCVs of Streptococcus tigurinus in a patient with a prosthetic joint infection. S. tigurinus is a novel pathogen of the Streptococcus mitis group and causes invasive infections. We sought to characterize S. tigurinus SCVs using experimental methods and find possible genetic explanations for their phenotypes. The S. tigurinus SCVs were compared with the wild-type (WT) isolate using phenotypic methods, including growth under different conditions, autolysis, and visualization of the cell ultrastructure by use of transmission electron microscopy (TEM). Furthermore, comparative genome analyses were performed. The S. tigurinus SCVs displayed reduced growth compared to the WT and showed either a very stable or a fluctuating SCV phenotype. TEM analyses revealed major alterations in cell separation and morphological abnormalities, which were partially explained by impaired autolytic behavior. Intriguingly, the SCVs were more resistant to induced autolysis. Whole-genome sequencing revealed mutations in the genes involved in general cell metabolism, cell division, stringent response, and virulence. Clinically, the patient recovered after a 2-stage exchange of the prosthesis. Comparative whole-genome sequencing in clinical strains is a useful tool for identifying novel genetic signatures leading to the most persistent bacterial forms. The detection of viridans streptococcal SCVs is challenging in a clinical laboratory due to the small colony size. Thus, it is of major clinical importance for microbiologists and clinicians to be aware of viridans streptococcal SCVs, such as those of S. tigurinus, which lead to difficult-to-treat infections.

Small colony variants (SCVs) of Staphylococcus aureus--a bacterial survival strategy

Small colony variants (SCVs) of Staphylococcus aureus have been implicated in chronic recurrent infections and have therefore gained renewed interest during the last decade. Moreover, SCVs have been shown to be part of the regular growth cycle, are highly dynamic or stable and can be selected during various harsh conditions. As such, the emergence of SCVs has been described not only in human, but also in veterinary medicine as well as in food microbiology. SCVs are characterized by impaired growth, down-regulation of genes for metabolism and virulence, while sigB and genes important for persistence and biofilm formation are up-regulated. Furthermore, SCVs are resistant to various antibiotics such as aminoglycosides, trimethoprim-sulfamethoxazole, fluorquinolones, fusidic acid or even to antiseptics such as triclosan. An underlying mechanism has been determined for hemin-, menadione- and thymidine-dependent SCVs as well as for SCVs which are impaired in their stress response. SCVs are optimized for persistence in the host. They are able to reverse and thereby constitute a highly dynamic subpopulation of S. aureus. Such phenotype switching constitutes an integral part of the infection process enabling the bacteria to hide inside the host cell without eliciting a strong host response.

Pacemaker lead infection and related bacteraemia caused by normal and small colony variant phenotypes of Bacillus licheniformis

Here, we report what we believe to be the first case of bacteraemia with small colony variants of Bacillus licheniformis related to a pacemaker lead infection by B. licheniformis displaying the normal phenotype. Arbitrarily primed PCR analysis showed a clonal strain. The infection was cured after the removal of the infected device.

Antimicrobial susceptibility and biofilm formation of Staphylococcus epidermidis small colony variants associated with prosthetic joint infection

We determined the frequency of isolation of non-aureus staphylococcal small colony variants (SCVs) from 31 patients with staphylococcal prosthetic joint infection (PJI) and described the antimicrobial susceptibility, auxotrophy, and biofilm-forming capacity of these SCVs. Eleven non-aureus SCVs were recovered, all of which were Staphylococcus epidermidis, and none of which was auxotrophic for hemin, menadione, or thymidine. Aminoglycoside resistance was detected in 5. Two were proficient, and 7 were poor, biofilm formers. With passage on antimicrobial free media, we observed a fluctuating phenotype in 3 isolates. We also noted a difference in antimicrobial susceptibility of different morphology isolates recovered from the same joints despite similar pulsed-field gel electrophoresis patterns. Our findings suggest S. epidermidis SCVs are common in PJI, and while they have a similar appearance to S. aureus SCVs, they do not necessarily share such characteristics as aminoglycoside resistance; auxotrophy for hemin, menadione, or thymidine; or enhanced biofilm formation. We also underscore the importance of antimicrobial susceptibility testing of all morphologies of isolates recovered from PJI.