Infection due to coagulase-negative staphylococci: Epidemiology, microbiology, and pathogenesis

INTRODUCTION — Coagulase-negative staphylococci (CoNS) are among the most frequent constituents of normal skin flora [1,2]. These organisms are increasingly recognized as agents of clinically significant infection, including bacteremia and endocarditis [3]. However, they are also common contaminants in clinical specimens; determining whether a clinical isolate is a contaminant or significant pathogen can be challenging.

The epidemiology, microbiology, and pathogenesis of CoNS will be reviewed here. Issues related to clinical manifestations and treatment of CoNS infections are discussed separately. (See "Infection due to coagulase-negative staphylococci: Clinical manifestations" and "Infection due to coagulase-negative staphylococci: Treatment".)

EPIDEMIOLOGY — CoNS are the most common cause of nosocomial bloodstream infections [4,5]. Among patients with blood cultures positive for CoNS, the fraction with a significant bloodstream infection has been found to range from 12 to 25 percent of cases; the remainder are identified as contaminants [6-9]. Such a distinction is important for clinical management. (See 'Distinguishing infection from contamination' below.)

Patients at particular risk for CoNS infection include those with prosthetic devices (eg, pacemakers, intravascular catheters, prosthetic heart valves, orthopedic implants) and immunocompromised hosts.

MICROBIOLOGY

Species — There are currently 47 species recognized in the genus Staphylococcus [2]. Staphylococci are aerobic and facultatively anaerobic gram-positive cocci that produce catalase and have a tendency to form irregular clusters. CoNS are not motile, and they do not form spores. Staphylococcus aureus and the several members of the Staphylococcus hyicus–intermedius group comprise the coagulase-positive staphylococcal species, while all remaining staphylococcal species are classified as coagulase negative. Occasional cases of CoNS infections are identified to be due to S. aureus when genetic testing is performed.

Staphylococcus epidermidis accounts for more than half of staphylococci isolated from human skin [10,11] and more than 75 percent of CoNS found in clinical specimens [12-14]. Other clinically significant species include S. saprophyticus, which causes urinary tract infections in young adult women [15], and S. lugdunensis, which has increasingly been recognized as a cause of invasive infections that include endocarditis, osteomyelitis, and sepsis [16]. (See "Staphylococcus lugdunensis".)

S. hominis, S. haemolyticus, S. capitis, S. warneri, and S. simulans are infrequently isolated as pathogens [2,12]. S. pettenkoferi is a relatively newly described species isolated from clinical specimens and implicated as a cause of bloodstream infections and osteomyelitis [17-20]. S. schleiferi includes both coagulase-negative (subsp schleiferi) and coagulase-positive (subsp coagulans) subspecies. It is a veterinary pathogen (associated with skin and ear infections in dogs) and has been reported as an infrequent cause of disease in humans, including wound or surgical site infections as well as endocarditis and meningitis [21,22].

Identification — CoNS are the most frequent blood culture isolates; they are also ubiquitous skin commensals and are common contaminants. For this reason, identification beyond the level of species to actual strain delineation can be employed to help address whether or not a particular isolate caused a true bacteremia [6,23]. Repetitive isolation of the same strain from an individual patient increases the probability of clinical significance [24]. Isolation of the same strain from more than one patient may also signal nosocomial transmission from a common source, such as patient to patient or staff to patient.

Species identification of CoNS may be approached using a combination of morphologic assessment (colony size and pigmentation), biochemical and metabolic testing, and antibiotic susceptibility results [25-27]. These assays are employed in several commercial kits, which allow for rapid species and subspecies identification within several hours to one day, with an accuracy of 70 to 90 percent [28-31]. Species misidentification may occur due to the large number of species that must be distinguished, some variability in expression of biochemical phenotypes among the non–S. epidermidis isolates, and ambiguity in interpreting colorimetric test results [32,33].

Nucleic acid–based techniques are increasingly being used for species identification of S. epidermidis and a number of other CoNS species [33,34]. Molecular techniques include polymerase chain reaction (PCR) amplification and sequencing of genes such as the 16S and 23S ribosomal DNA genes and their internal transcribed spacer regions, the manganese-dependent superoxide dismutase gene (sodA), the elongation factor TU gene (tuf), and the RNA polymerase beta subunit gene (rpoB) [2,35-38] as well as chromosomal analysis by ribotyping (based upon sequence differences in genes encoding rRNAs and flanking DNA), restriction fragment length polymorphism analysis, pulsed-field gel electrophoresis (allowing for resolution of large DNA fragments by restriction endonucleases that cleave DNA infrequently), and random amplified polymorphic DNA assays [12,25,39].

Increasingly, bacterial whole-genome sequencing using next-generation sequencing (NGS) technologies is being employed in epidemiologic outbreak investigations, analysis of virulence determinants, and in silico antimicrobial susceptibility predictions, including application to the CoNS [40-44]. However, although NGS is especially valuable in epidemiologic analyses due to the enhanced taxonomic resolution it provides compared with conventional methods [42], significant challenges remain in terms of applying these technologies as part of routine diagnostics in the clinical laboratory. These include not only costs and turnaround times, which continue to improve, but especially the development of automated bioinformatics pipelines for data analysis, advances in data management and sharing, and standardization and validation of procedures used to ensure quality control [40,45-49].

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), a technique that profiles the mass/charge ratios of protein fragments released from the bacterial cell surface to produce characteristic spectra, has emerged as a powerful tool for bacterial identification that is both rapid and cost-effective [50,51]. Several studies have compared MALDI-TOF MS with automated identification systems based on bacterial phenotype and with PCR-based sequencing methods for the identification of CoNS. Overall, MALDI-TOF MS was found to have a very high diagnostic accuracy of approximately 93 to >99 percent for the species identification of a broad array of both clinical and reference isolates of CoNS [52-54]. A study using biomarker-based MALDI-TOF MS, focusing on detection of ribosomal proteins, reported CoNS to be correctly identified to the species level with a sensitivity of 100 percent by biomarker MALDI-TOF MS compared with 77.5 percent by conventional automated biochemical methods when a total of 48 clinical isolates were examined [55]. A MALDI-TOF MS analysis of CNS isolates identified S. lugdunensis as a more common isolate than previously recognized using other speciation techniques [56]. (See "Staphylococcus lugdunensis".)

Distinguishing infection from contamination — Distinguishing infection from contamination can be challenging. (See "Detection of bacteremia: Blood cultures and other diagnostic tests", section on 'Assessing clinical significance'.)

Microbiologic findings supportive of infection include [57-59]:

●Growth in both aerobic and anaerobic blood culture bottles

●Multiple cultures (≥2 blood culture sets obtained from separate venipunctures) positive for the same organism, with identical antibiograms and/or molecular evidence of genetically identical strains

However, contamination may be present even if CoNS is recovered from ≥2 blood culture sets [58]. In addition, molecular studies may demonstrate presence of more than one strain, even in the setting of true infection [60]; in such cases, clinical infection may be attributable to polyclonal involvement by genetically distinct isolates.

PATHOGENESIS — Much of the work on the pathogenesis of CoNS infections derives from the observation that most infections with these organisms occur in the setting of prosthetic devices. CoNS possess determinants that facilitate survival on skin surfaces, biofilm formation, adhesion to tissue and prosthetic surfaces, and components involved in immune evasion [61,62]. The surface molecule poly-gamma-DL-glutamic acid appears to play an important role in colonization of skin surfaces and immune evasion [63].

Prosthetic devices — Prosthetic devices are presumably inoculated with small numbers of CoNS at the time of implantation. The organisms may originate from the flora of the patient, surgical personnel, or the environment. Intravascular catheters are colonized by skin flora that can migrate down the external portion of the catheter tunnel or colonize the hub with catheter manipulations, thereby gaining access to the internal surface of the catheter [12,64].

The biofilm that forms along the foreign body appears to facilitate infection by shielding organisms with relatively low virulence from elimination by host defenses or antimicrobial therapy [65], perhaps through changes in bacterial metabolism, creation of a permeability barrier, or alterations in leukocyte function [65-67].

Biofilm — The factors that make CoNS especially adept at surviving on various biomaterials include adherence and production of biofilm.

Attachment is thought to be mediated by nonspecific hydrophobic and van der Waals interactions. Foreign materials become coated with a layer of host proteins, including collagen, fibrinogen, fibronectin, and others, which serve as potential receptors for various staphylococcal surface proteins. Strains of S. epidermidis possess surface adhesins that are critical for interacting with these host proteins and mediating a more specific adherence [25,68-72].

Following attachment of the organism to the foreign material, an extracellular polysaccharide matrix is elaborated, which encases the bacteria. This biofilm may function as a barrier to antibiotic penetration and interfere with host defenses, including T lymphocyte activation, opsonization, polymorphonuclear leukocyte migration, and macrophage function [12,39]. Slime-producing strains of S. epidermidis exhibit greater virulence and disease-causing potential in patients with prosthetic devices [73,74], and infections with these organisms are more resistant to bacteriologic cure [75].

Biofilms also release bacteria into the bloodstream and potentially seed other tissue sites. Catecholamines such as norepinephrine facilitate organism growth and biofilm production on polystyrene and silicon, perhaps through increased acquisition of iron from transferrin [76]. A cell surface component that mediates the intercellular adhesion of S. epidermidis has been identified; it is known as polysaccharide intercellular adhesin (PIA), a linear b-1,6-linked glycosaminoglycan. It permits cell clustering and accumulation of multilayered biofilm [77]. Production of PIA is controlled by genes in the intercellular adhesion (ica) operon, including both biosynthetic and regulatory elements [78,79]. A variety of environmental stresses stimulate expression of the ica operon, including oxygen limitation, high temperature, osmotic stress, and sublethal concentrations of certain antibiotics [78], suggesting that biofilm formulation may represent an adaptation to hostile environmental conditions encountered within the host. A specific class of surfactant peptides, the beta-type of phenol-soluble modulins (PSMs), have been shown to play a role in the maturation and detachment of S. epidermidis biofilms, factors that may contribute to the dissemination of infection [80,81].

Biofilm also appears to contribute to antibiotic tolerance. In a report of a pacemaker-associated endocarditis due to a susceptible S. epidermidis, the isolate was found to develop antibiotic tolerance over time. The isolate increased biofilm formation and reduced its growth rate [82].

In addition to biofilm production, other potential virulence factors of CoNS include the various exoproteins hemolysins, DNase, protease, and fibrinolysin [13]. Aggressive and highly cytolytic toxins have generally been thought to be absent from the CoNS [61]. However, a PSM family member, the methicillin resistance island-encoded peptide toxin termed PSM-mec, was shown to play a significant role in mediating sepsis due to methicillin-resistant S. epidermidis; this was established by studying parental strains and isogenic mutants in a combination of tissue culture and mouse-infection models [83].

S. epidermidis clones may have unique features, including antimicrobial resistance genes, which enable successful spread in health care settings. Use of whole genome sequencing has demonstrated that S. epidermidis transmission in health care settings occurs more frequently than previously suspected [42,84]. A study of transmission identified clonal lineages of S. epidermidis as responsible for bacteremias in a neonatal intensive care unit [42].

There are limited data on the host response to CoNS infections, especially those involving prosthetic devices. The host response is clearly not sufficient to protect against infection. The efficiency of phagocytic killing in the presence of biofilm is diminished [85]. The role of complement or adaptive immune mechanisms is not well defined.

In the appropriate setting, S. epidermidis may serve as a reservoir for the transfer of virulence-related genetic information to S. aureus, such as transfer of the arginine catabolic mobile element to the USA300 clone of methicillin-resistant S. aureus [86]. Methicillin-resistant S. epidermidis clones responsible for healthcare-associated infections have been found to express an accessory genetic element, tarIJLM, which leads to production of an S. aureus-type of surface-exposed wall teichoic acid (WTA) [87]. This altered WTA increased the virulence of S. epidermidis in an animal model of invasive infection and also rendered the bacteria susceptible to infection by bacteriophages that normally target S. aureus providing a mechanism for exchange of DNA between the species which could lead to transfer of virulence and colonization factors [87].

On the skin, CoNS appear to play an important probiotic role in maintaining stability of the normal skin flora, priming the immune system, and preventing colonization by other more invasive pathogens such as S. aureus [88-90]. For instance, S. epidermidis strains secreting the serine protease Esp were shown to interfere with S. aureus biofilm formation and colonization of the nasal mucosa by a mechanism that remains to be fully defined [91]. Strains of CoNS have also been shown to secrete antimicrobial peptides (AMPs) which kill S. aureus and act in synergy with the human AMP LL-37 [92]. CoNS isolates from individuals with atopic dermatitis (AD) were less likely to produce this antimicrobial activity, especially when the individuals were S. aureus culture-positive, while application of antimicrobial CoNS strains to individuals with AD was found to suppress S. aureus colonization [92,93].

SUMMARY

●Coagulase-negative staphylococci (CoNS) are the most frequent constituent of the normal flora of the skin. These organisms can be agents of clinically significant infection but are also common contaminants in clinical specimens. (See 'Introduction' above.)

●Staphylococcus epidermidis accounts for more than half of staphylococci isolated from human skin and more than 75 percent of CoNS in clinical specimens. Other clinically significant species include S. saprophyticus and S. lugdunensis. (See 'Microbiology' above.)

●Use of intravascular devices is an important contributor to bloodstream infections with CoNS. CoNS account for approximately one-third of bloodstream isolates in intensive care units, making these organisms the most common cause of nosocomial bloodstream infection. (See 'Epidemiology' above.)

●Among patients with blood cultures positive for CoNS, approximately 12 to 25 percent have significant bloodstream infection. (See 'Epidemiology' above.)

●Distinction between contamination and true infection is important for clinical management. Microbiologic factors favoring infection over contamination include growth in culture within 48 hours, growth in both aerobic and anaerobic bottles, and multiple cultures (two or more blood culture sets) positive for the same organism with identical antibiograms and/or molecular evidence of genetically identical strains. (See 'Distinguishing infection from contamination' above.)

●Species identification may be approached using a combination of morphologic assessment, biochemical and metabolic testing, antibiotic susceptibility results, nucleic acid-based techniques, and MALDI-TOF MS. Identification beyond the level of species (to actual strain delineation) can address whether a particular isolate caused a true bacteremia. Repetitive isolation of the same strain from an individual patient increases the probability of clinical significance. (See 'Identification' above.)

●CoNS possess determinants that facilitate survival on skin surfaces, biofilm formation, adhesion to tissue and prosthetic surfaces, and components involved in immune evasion. (See 'Pathogenesis' above.)