New Insights into the Prevention of Staphylococcal Infections and Toxic Shock Syndrome

Ying-Chi Lin; Marnie L Peterson

Expert Rev Clin Pharmacol. 2010;3(6):753-767.

Five-year View

We speculate that the field of S. aureus and TSS research will evolve considerably over the next 5–10 years. With several (active and passive) vaccine candidates in clinical trials currently, it is possible a S. aureus vaccine may be available for individuals at risk by the end of the next decade. The prevalence of MRSA in the hospital and community setting will most likely continue to rise, which will increase the need for real-time diagnostics to inform the clinician of antimicrobial resistance and the presence of specific superantigens or cytolysins so optimal therapy can be utilized. Owing to continued increase in resistance and availability of newer antibiotics with less toxicity, vancomycin will most likely no longer be the antimicrobial of choice for MRSA infections in the hospital setting.

Therapeutics Targeted at the Effects of Exotoxins on Mucosal & Skin Surfaces

Despite these advances, the study of the pathogenesis of S. aureus will continue to evolve with an overall focus on the role of exotoxins and their effects on mucosal and skin surfaces, that is, the site of infection. Recently, a study described the role of metalloprotease 10 (ADAM10) on host cells, which interacts with α-toxin and is required to initiate the cytolytic pore and inflammatory effects of α-toxin on mammalian cells.^[99] This is the first study identifying a host receptor for α-toxin, despite decades of research describing the effects of this toxin on mammalian cells, and this finding will most likely lead to new therapeutic anti-staphylococcal agents. Likewise, superantigens have been reported to induce changes in epithelial cellular morphology and secretion of proinflammatory cytokines/chemokines from vaginal, bronchial, nasal and intestinal cells.^[100–103] Although the host receptor(s) for superantigens has yet to be determined, the ability of a topical agent, GML, to reduce exotoxin production and overall proinflammatory cytokines at the site of infection and increase survival in a rabbit model of TSS,^[85] suggests anti-inflammatory effects on mucosal surfaces are an important consideration in the development of therapeutics.

An overall understanding of the exoproteins most important in the ability of S. aureus to cause infection and the corresponding host immunological responses (i.e., mucosal response) is critical and will open new areas of research. New therapeutic agents are being developed currently to support this area of research, including anti-toxin production and neutralizing agents, and/or receptor antagonists, mucosal anti-inflammatory agents and vaccines derived from toxoids.

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Financial & competing interests disclosure
Marnie Peterson's research is supported by grants from NIH: National Institute of Allergy and Infectious Diseases (RO1AI073366), Powell Center for Women's Health and University of Minnesota Academic Heath Center Office of the Vice President, and 3M. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.
No writing assistance was utilized in the production of this manuscript.

Cite this: New Insights into the Prevention of Staphylococcal Infections and Toxic Shock Syndrome - Medscape - Nov 01, 2010.

Table 1. Secreted *Staphylococcus aureus* virulence factors.
Secreted virulence factor	Putative function
Toxic shock syndrome toxin 1; staphylococcal enterotoxins; staphylococcal enterotoxin-like toxins	Activate T cells and macrophages
Cytolysins (α-, β-, γ-, δ-toxins); phenol-soluble modulin-like peptides; leukocidins (PVL, LukD/E)	Induce apoptosis (at low concentration) and lysis of various cell types, including erythrocytes, lymphocytes, monocytes, epithelial cells; target specificity varies
Lipase	Inactivate fatty acids
Hyaluronidase	Degradation of hyaluronic acid
Serine proteases; cysteine proteases (including staphopains); aureolysin	Inactivate neutrophil proteolytic activity; inactivate antimicrobial peptides
Staphylokinase	Plasminogen activation; inactivate antimicrobial peptides
Exfoliative toxins	Act as serine proteases; activate T cells
Chemotaxis inhibitory protein of Staphylococccus aureus; Staphylococcal inhibitor of complement	Inhibit complement
Staphylococcal superantigen-like proteins; extracellular adherence protein	Inhibit complement C5 and IgA; inhibit neutrophil migration

PVL: Panton–Valentine leukocidin

Table 2. CDC case definition of staphylococcal toxic shock syndrome.
Criteria	Definition
*Clinical*
Fever	Temperature greater than or equal to 102.0°F (≥38.9°C)
Rash	Diffuse macular erythroderma
Desquamation	1–2 weeks after onset of illness, particularly on the palms and soles
Hypotension	Systolic blood pressure ≤90 mmHg for adults or <5th percentile by age for children aged <16 years; orthostatic drop in diastolic blood pressure ≥15 mmHg from lying to sitting, orthostatic syncope or orthostatic dizziness
*Multisystem involvement (≥3 organ systems)*
Gastrointestinal	Vomiting or diarrhea at onset of illness
Muscular	Severe myalgia or creatine phosphokinase level at least twice the upper limit of normal
Mucosal	Mucous membrane: vaginal, oropharyngeal or conjunctival hyperemia
Renal	Blood urea nitrogen or creatinine at least twice the upper limit of normal for laboratory or urinary sediment with pyuria (≥5 leukocytes per high-power field) in the absence of urinary tract infection
Hepatic	Total bilirubin, alanine aminotransferase enzyme or spirate aminotransferase enzyme levels at least twice the upper limit of normal for laboratory
Hematologic	Platelets <100,000/mm³
CNS	Disorientation or alterations in consciousness without focal neurologic signs when fever and hypotension are absent
*Laboratory*
Culture	If obtained, negative results on blood, throat, or cerebrospinal fluid cultures (blood culture may be positive for Staphylococcus aureus)
Titer	If obtained, no rise in titer to Rocky Mountain spotted fever, leptospirosis or measles
*Case classification*
Probable	A case which meets the laboratory criteria and in which four of the five clinical findings described above are present
Confirmed	A case which meets the laboratory criteria and in which all five of the clinical findings described above are present, including desquamation, unless the patient dies before desquamation occurs

Data from [201].

Table 3. Antistaphylococcal therapies and effects on exotoxins.
Classification	Antimicrobial agents	Mechanism of action	Main resistance mechanism(s)	Effect on exotoxin at subgrowth inhibitory concentrations
β-lactam	Methicillin, oxacillin and cephalosporins	Inhibits cell wall biosynthesis	β-lactamase (penicillinase); modified penicillin binding protein PBP2a (mecA)	↑
Glycoprotein	Vancomycin	Inhibits cell wall synthesis by inhibiting the incorporation of newly synthesized precursors in the cell wall peptidoglycan	Thickened cell wall that traps vancomycin (increased and structurally varied pentapeptides); change of binding site (D-ala-D-ala to D-ala-D-Lac)	↔
Lipopepetide	Daptomycin	Depolarization of the membrane	Decreased outer membrane permeability	NA
Macrolide	Erythromycin, clarithromycin and azithromcin	Reversibly binds to 23S ribosomal RNA (rRNA) of the 50S subunit of bacterial ribosome inhibiting RNA-dependent protein synthesis	A methylase that dimethylates an adenine within the 23S ribosomal binding site of the macrolides (erm; inducible); an efflux pump (mef [E]) that expels the macrolides	↓
Lincosamide	Clindamycin	Binds to the 50S ribosome and inhibits protein synthesis	erm; cross-resistance to macrolides	↓↓↓
Oxazolidinone	Linezolid	Binds to 23S rRNA of the 50S ribosomal subunits and prevents formation of the 70S initiation complex	Mutation(s) in the 23S rRNA	↓↓↓
Streptogramin	Quinupristin/dalfopristin	Inhibits protein synthesis	erm; cross-resistance to macrolides and clindamycin	NA
Aminoglycoside	Gentamicin, streptomycin, kanamycin and amikacin	Binds to the 30S ribosome and inhibits protein synthesis	Aminoglycoside-modifying enzymes	↓
Tetracycline	Tetracycline, minocycline, doxycycline and tigecycline	Binds to the 30S ribosome and inhibits protein synthesis	Efflux transporters	↓
Sulfonamide	Trimethoprim–sulfamethoxazole	Inhibits folic acids biosynthesis	Single amino acid substitution in dihydrofolate reductase	NA

NA: Not available.

Table 4. Experimental agents for prevention of staphylococcal TSS/exoprotein toxicity.
Reagent	Mechanisms of action	Supporting evidence
GML	Inhibit the growth of Staphylococcus aureus Delay the production of S. aureus exoproteins Immunomodulation effects on mammalian cells via membrane stabilization	In vitro: GML reduces the production of proinflammatory cytokines and chemokines by epithelial cells in response to S. aureus and purified TSST-1[81,82] In vivo: GML, as 5% vaginal gel, prevents lethality in rabbits challenged vaginally with purified TSST-1[81] Tampon coated with GML reduces S. aureus growth, exotoxin production and vaginal IL-8 secretion [104]
Hemoglobin subunit inhibitors	Target two-component and quorum sensing systems to inhibit exoprotein production [87]	In vitro: mixtures of α and β hemoglobin (≥1 µg/ml) inhibit S. aureus exoproteins[87] In vivo: TSST-1 and α-toxin were only detected in tampon sections containing little or no menstrual blood, despite the high bacterial counts [86]
Vβ peptides (SEB antagonists)	A synthesized immunoglobulin-like peptide competes with the particular TCR binding site to prevent SEB-mediated T-cell activation and lethality in rabbits intravenously administered with SEB [88]	In vivo: rabbits injected with Vβ protein (dose ≥32.5 µg/kg) survived through endotoxin enhancement model of TSS[88] The protective capacity of the Vβ agent was 2000-times greater than that of IVIG

GML: Glycerol monolaurate.

Table 5. Experimental staphylococcal vaccines and immunotherapies in clinical trials.
Bacterial target	Putative role in virulence	Proposed strategy (product/company)	Trial conclusion
Capsule polysaccharide (CP5 and CP8)	Avoidance of phagocytosis	Active immunization (StaphVAX®/Nabi)	Vaccine efficacy (57%) only last up to 40 weeks after immunization in end-stage renal disease (ESRD) hemodialysis patients (n = 1804). However, no significant protection was detected against bacteria in a confirmatory follow-up trial (n = 3600)
		Human polyclonal antiserum (Altastaph®/Nabi)	No reduction in preventing Staphylococcus aureus bacteremia in very low-birth weight (<1500 g) neonates (n = 206) No treatment effect in children ≥7 years with S. aureus bacteremia and persistent fever (n = 40)
Clumping factor A	Attachment to fibrinogen and biomaterial surfaces	Selected IVIG (INH-A21; Veronate®/Inhibitex)	No prevention in mortality or the rates of late-onset sepsis in infants (n = 1983)
		Humanized monoclonal antibody (tefibazumab; Aurexis®/Inhibitex)	No difference in composite clinical end points (a relapse, a complication, or death of bacteria) in hospitalized adult patients with bacteremia as an add-on therapy to standard therapy (n = 63)
ATP-binding cassette transporter	Nutrient uptake and cell attachment	Human-derived single-chain variable antibody fragment (Aurograb®/NeuTec)	Lack of efficacy as add-on therapy to vancomycin to treat deep-seated MRSA infections in Phase II trial (development terminated)
Lipoteichoic acid^†	Bacteria cell wall component	Humanized monoclonal antibody (pagibaximab/Biosynexus)	Pilot study showed reduced staphylococcal sepsis rate in very low-birth weight neonates (n = 88) Safety and efficacy for prevention of staphylococcal sepsis in very low birth weight neonates (Phase II/III) is in progress (NCT00646399)
Iron-regulated surface determinant B^†	Iron uptake	Active immunization (V710/Merck)	Safety and immunogenicity study in 198 patients with ESRD on chronic hemodialysis was completed Feburary 2010 Safety and efficacy in prevention of serious S. aureus infections in adult patients within 90 days after selective cardiothoracic surgery is in process (NCT00518687)
SA3Ag^†	MSCRAMMs	Active immunization (Pfizer)	Phase I trial is in progress (NCT01018641)
Staphylococcus aureus toxoids (α-toxin and LukS-PV)^†	Staphylococcal cytotoxic exoproteins	Active immunization (Nabi)	Phase I/II trial is in progress (NCT01011335)
Staphylococcal enterotoxin B^†	Toxic shock syndrome	Active immunization (STEBVax/NIAID)	Phase I is in progress (NCT00974935)

^†There are active clinical trials investigating the product.
MSCRAMM: Microbial surface components recognizing adhesive matrix molecules; NIAID: The National Institute of Allergy and Infectious Disease.
Data from [106,202].

Sidebar

Key Issues

Methicillin-resistant Staphylococcus aureus(MRSA) isolates account for more than 60% of nosocomial S. aureusinfections in the intensive care units and more than 70% of S. aureusskin and soft-tissue infections in the community.
Hospital acquired (HA)-MRSA and community acquired (CA)-MRSA isolates have distinct exotoxin profiles. Panton–Valentine leukocidin (PVL) is epidemiologically associated with CA-MRSA isolates. Staphylococcal enterotoxin B (SEB) and SEC, have also been associated with CA-MRSA.
S. aureusproduces an array of virulence factors to facilitate its pathogenesis. Initially researchers focused on the role of cell surface virulence factors, such as capsule; however, recently researchers have started to recognize the overall importance of staphylococcal exoproteins, such as cytolysins and superantigens, in the initiation and progression of infections via direct tissue damage to mucosal membranes and skin.
Antibiotics of choice for treating S. aureus-related illnesses, β-lactams for methicillin-susceptible S. aureus(MSSA) and vancomycin for MRSA, kill the organism by lysis of the cell wall or inhibit cell wall biosynthesis, but are not able to inhibit S. aureusexoprotein production or neutralize the existing toxins' effects on host cells. β-lactams may even induce production of cytolysins and other virulence-related exoproteins when inadequately used for treating MRSA, which potentially worsens clinical outcomes.
Protein-synthesis inhibition is important in the selection of antimicrobial agents to treat infections caused by toxin-producing Gram-positive pathogens. Linezolid and clindamycin have proven efficacious in reducing exotoxin production even at subgrowth inhibitory concentrations.
Intravenous immunoglobulin (IVIG) has been used to treat toxic shock syndrome due to its ability to neutralize superantigens. However, clinical benefits of such treatment in staphylococcal toxic shock syndrome (TSS) may be unproven. New anti-toxin therapeutic approaches such as hypersera, V β antagonists, hemoglobin subunit inhibitors and glycerol monolaurate are in development for TSS prevention and as treatments.
Vaccination targets to date have been cell-surface targets, including iron-regulated surface determinant B (IsdB), but exotoxins (PVL and α-toxin) and the superantigen (SEB) are being evaluated in clinical trials as vaccine targets.
Future areas of research include a focus on the exotoxins such as cytolysins and superantigens and their effects on mucosal surfaces; mucosal receptor antagonists, anti-inflammatory agents and a multivalent approach to vaccination that includes the cytolysins and superantigens.