Endemic, epidemic and pandemic infections: the roles of natural and acquired herd immunity.

PURPOSE OF REVIEW: This review summarizes the general concepts of innate and acquired immunity, including vaccine use and hesitancy, as they relate to reduction of the global burden of highly communicable infectious diseases. RECENT FINDINGS: Vaccination to increase herd immunity remains the cornerstone of disease prevention worldwide yet global vaccination goals are not being met. Modern obstacles to vaccine acceptance include hesitancy, reduced altruistic intentions, impact of COVID-19, distrust of science and governmental agencies as well as recent geopolitical and environmental disasters. Together, such barriers have negatively impacted immunization rates worldwide, resulting in epidemics and pandemics of serious life-threatening infections from vaccine-preventable diseases, especially those affecting children. In addition, pathogens thought to be controlled or eradicated are reemerging with new genetic traits, making them more able to evade natural and acquired immunity, including that induced by available vaccines. Lastly, many serious and widespread infectious diseases await development and utilization of efficacious vaccines. SUMMARY: The global burden of communicable diseases remains high, necessitating continued pathogen surveillance as well as vaccine development, deployment and continued efficacy testing. Equally important is the need to educate aggressively the people and their leaders on the benefits of vaccination to the individual, local community and the human population as a whole.

Investigating the immunomodulatory activities of omadacycline.

BACKGROUND: Apart from their antimicrobial activities, some antibiotics have immunomodulatory effects on host cells, particularly monocytes. Because hyperactivation of the pro-inflammatory cytokine response contributes to acute lung injury in patients with bacterial pneumonia and other lung diseases, antimicrobial agents with immunomodulatory activity can reduce cytokine-mediated tissue injury and improve outcomes. OBJECTIVES: Omadacycline has been recently FDA-approved for community-acquired bacterial pneumonia and acute bacterial skin and skin-structure infections. The present study investigated omadacycline's ability to modulate LPS-induced production of pro-inflammatory cytokines (TNF-α, IL-1ß), acute-phase reactants (IL-6) and anti-inflammatory cytokines (IL-4, IL-10) by human monocytes in vitro. METHODS: Isolated human monocytes from healthy consenting adults were cultured in RPMI with 1% pooled human serum. Cells were pre-exposed to omadacycline (0.5-64 µg/mL), minocycline (25, 50 or 25 µg/mL) or azithromycin (20, 40 or 80 µg/mL) for 2 h, followed by stimulation with Escherichia coli LPS for 24 h. Cytokines elaborated in the culture supernatant were quantitated by multiplex immunoassay. RESULTS: Omadacycline dose-dependently suppressed LPS-induced production of all cytokines tested. Only high-dose minocycline (100 µg/mL) modestly suppressed TNF-α whereas minocycline significantly increased LPS-induced IL-1ß production. Lower concentrations of minocycline were also stimulatory for IFN-γ, IL-6 and IL-4. Except for suppression of IL-6, azithromycin was largely without effect. CONCLUSIONS: Omadacycline has unique and broad immunomodulatory properties. Such activity supports its use in settings where hyperactivation of the immune response contributes to tissue injury and poor outcomes, especially at sites where pro-inflammatory M-type 1 macrophages dominate the cellular immune response.

Streptococcus pyogenes NAD+-Glycohydrolase Reduces Skeletal Muscle ßNAD+ Levels Independently of Streptolysin O.

Necrotizing soft tissue infections caused by Streptococcus pyogenes (group A streptococcus [GAS]) are characterized by rapid and extensive necrosis of fascia and muscle. Molecular epidemiological studies have demonstrated a positive correlation between GAS isolates that cause invasive infections and the production of S. pyogenes NAD+-glycohydrolase (SPN), an NADase secreted by GAS, but the effect of SPN on muscle cells has not been described. Thus, using standard ßNAD+ and ATP quantification assays, we investigated the effects of SPN on cultured human skeletal muscle cell (SkMC) ßNAD+ and ATP with and without streptolysin O (SLO)-a secreted cholesterol-dependent cytolysin known to act synergistically with SPN. We found that culture supernatants from GAS strains producing SLO and SPN depleted intracellular ßNAD+ and ATP, while exotoxins from a GAS strain producing SLO and an enzymatically-inactive form of SPN had no effect on ßNAD+ or ATP. Addition of purified, enzymatically-active SPN to NADase-negative culture supernatants or sterile media reconstituted ßNAD+ depletion but had no effect ATP levels. Further, SPN-mediated ßNAD+ depletion could be augmented by SLO or the homologous cholesterol-dependent cytolysin, perfringolysin O (PFO). Remarkably, SPN-mediated ßNAD+ depletion was SkMC-specific, as purified SPN had minimal effect on epithelial cell ßNAD+. Taken together, this study identifies a previously unrecognized role for SPN as a major disruptor of skeletal muscle ßNAD+. Such activity could contribute to the rapid and widespread myonecrosis characteristic of severe GAS soft tissue infections.

Necrotizing Soft Tissue Infections.

Necrotizing soft tissue infections occur after traumatic injuries, minor skin lesions, nonpenetrating injuries, natural childbirth, and in postsurgical and immunocompromised patients. Infections can be severe, rapidly progressive, and life threatening. Survivors often endure multiple surgeries and prolonged hospitalization and rehabilitation. Despite subtle nuances that may distinguish one entity from another, clinical approaches to diagnosis and treatment are highly similar. This review describes the clinical and laboratory features of necrotizing soft tissue infections and addresses recommended diagnostic and treatment modalities. It discusses the impact of delays in surgical debridement, antibiotic use, and resuscitation on mortality, and summarizes key pathogenic mechanisms.

Emerging erythromycin and clindamycin resistance in group A streptococci: Efficacy of linezolid and tedizolid in experimental necrotizing infection.

OBJECTIVES: Clindamycin (CLI) and erythromycin (ERY) resistance is increasing among group A streptococci (GAS) causing invasive disease and alternative treatments are urgently required. In this study, the efficacy of the newer oxazolidinone tedizolid (TZD) was compared with the first drug in this class, linezolid (LNZ), in experimental murine myonecrosis caused by ERY-susceptible/CLI-susceptible (ERYS/CLIS) or ERY- resistant/CLI-resistant (ERYR/CLIR) GAS. METHODS: Normal adult outbred Swiss Webster female mice (10 per group) were infected intramuscularly with ERYS/CLIS (ATCC 12384) or ERYR/CLIR (15-003) GAS. Treatments began 4 h post-infection and continued for 72 h. TZD and LNZ (10, 20 and 40 mg/kg) were given intraperitoneally every 12 h. Saline, penicillin (PEN), CLI and ERY were given every 6 h. Survival and infection severity signs and symptoms were followed for 12 days. RESULTS: Both GAS strains were susceptible to LNZ, TZD and PEN; strain 15-003 was confirmed as constitutively resistant to ERY and CLI. Blood levels following a 40 mg/kg dose of LZD and TZD were 30.9 ± 4.0 µg/mL and 21.9 ± 5.3 µg/mL, respectively. Both TZD and LNZ were highly efficacious for the treatment of severe experimental myonecrosis caused by ERYS/CLIS and, importantly, ERYR/CLIR GAS. CONCLUSION: In the current era of emerging macrolide/lincosamide resistance among GAS, these data support the use of TZD and LNZ as first-line antibiotics for the treatment of life-threatening GAS infections in humans.

The effects of iclaprim on exotoxin production in methicillin-resistant and vancomycin-intermediate Staphylococcus aureus.

PURPOSE: Extracellular protein toxins contribute to the pathogenesis of Staphylococcus aureus infections. The present study compared the effects of iclaprim and trimethoprim - two folic acid synthesis inhibitors - with nafcillin and vancomycin on production of Panton-Valentine leukocidin (PVL), alpha haemolysin (AH) and toxic-shock syndrome toxin I (TSST-1) in methicillin-resistant and vancomycin-intermediate S. aureus (MRSA and VISA, respectively). METHODOLOGY: Northern blotting and RT-PCR were used to assess gene transcription; toxin-specific bioassays were used to measure protein toxin production. RESULTS: As shown previously, sub-inhibitory concentrations (sub-MIC) of nafcillin increased and prolonged MRSA toxin gene transcription and enhanced PVL, TSST-1 and AH production. Sub-inhibitory doses of iclaprim and trimethoprim delayed maximal AH gene (hla) transcription and suppressed AH production; both drugs delayed, but neither reduced, maximal TSST-1 production. Trimethoprim significantly increased lukF-PV expression and PVL production compared to both untreated and iclaprim-treated cultures. Higher concentrations of iclaprim and trimethoprim markedly suppressed MRSA growth, mRNA synthesis and toxin production. In VISA, iclaprim, vancomycin and nafcillin variably increased tst and hla expression, but only nafcillin increased toxin production. Despite its ability to increase hla expression, iclaprim was the most potent inhibitor of AH production. CONCLUSIONS: We conclude that, due to its ability to suppress toxin production, iclaprim should be effective against severe staphylococcal infections caused by toxin-producing MRSA and VISA strains, especially given its ability to concentrate at sites of infection such as skin and skin structures and the lung.

Subinhibitory Dalbavancin Attenuates Exotoxin Production from Methicillin-Sensitive and Methicillin-Resistant Staphylococcus aureus In Vitro.

This study investigated the effects of subinhibitory doses of the lipoglycopeptide antibiotic dalbavancin on Staphylococcus aureus toxin production in vitroS. aureus toxin production levels were compared to those seen with the natural glycopeptide antibiotic vancomycin and with representative beta-lactam and oxazolidinone antibiotics. While neither dalbavancin nor vancomycin adversely affected toxin production, of these glycopeptide antibiotics, only dalbavancin significantly attenuated toxin production at subinhibitory concentrations. These findings support the recent success of dalbavancin for treatment of staphylococcal infections.

Fidaxomicin reduces early toxin A and B production and sporulation in Clostridium difficilein vitro.

PURPOSE: Fidaxomicin, a macrocyclic antibiotic, has been approved for the treatment of Clostridium difficile infection (CDI). Previous work by our group has demonstrated that some antibiotics at sub-inhibitory concentrations stimulate early toxin production and sporulation by C. difficile. Prior studies revealed that fidaxomicin, when added to late stationary-phase organisms, reduced exotoxin production and spore formation by C. difficile. However, the ability of fidaxomicin to trigger early virulence factor production and spore formation has never been investigated. METHODOLOGY: Sub-inhibitory concentrations of the RNA synthesis inhibitor fidaxomicin (1/4×, 1/8×, 1/16× MIC) were added immediately to lag-phase cultures of historical (strain 9689) and epidemic BI/NAP1/027 (strain 5325) strains of C. difficile, and their effects on sporulation and toxin A (TcdA) and toxin B (TcdB) production were compared.Results/Key findings. Even at sub-inhibitory concentrations, all doses of fidaxomicin reduced both TcdA and TcdB gene expression and protein production in the historical and epidemic C. difficile strains. Fidaxomicin also dose-dependently reduced viable spore production by the 9689 and 5325 strains. Reductions in spore formation were also observed in both strains treated with tigecycline and vancomycin. However, all concentrations of metronidazole stimulated a ~2 log increase in spore production by the 5325 isolate. CONCLUSION: The ability of fidaxomicin to suppress early exotoxin production and endospore formation by historical and epidemic strains of C. difficile may explain its clinical success in treating severe and recurrent cases of CDI disease.

Spontaneous C. septicum gas gangrene: A literature review.

As the infectious disease paradigm undergoes a subtle shift, unusual infections associated with malignancy and immunosuppression are being increasingly reported. Spontaneous or non-traumatic Clostridium septicum infection is one such unusual infection which has gained prominence. This article aims to understand the pathophysiology, clinical manifestations and current trends in diagnosing and treating this rare but deadly infection. To understand the multifactorial causation of this infection a review of published cases of spontaneous C. septicum gas gangrene was performed and a total of 94 such cases were identified. Several factors were analyzed for each case: age, infection location and underlying illness, presenting signs and symptoms, neutropenia, gross pathology of the colon, antibiotic use, surgical intervention, and survival. A known or occult malignancy was present in 71% patients and an overall mortality of 67% was observed.

Effects of delayed NSAID administration after experimental eccentric contraction injury - A cellular and proteomics study.

BACKGROUND: Acute muscle injuries are exceedingly common and non-steroidal anti-inflammatory drugs (NSAIDs) are widely consumed to reduce the associated inflammation, swelling and pain that peak 1-2 days post-injury. While prophylactic use or early administration of NSAIDs has been shown to delay muscle regeneration and contribute to loss of muscle strength after healing, little is known about the effects of delayed NSAID use. Further, NSAID use following non-penetrating injury has been associated with increased risk and severity of infection, including that due to group A streptococcus, though the mechanisms remain to be elucidated. The present study investigated the effects of delayed NSAID administration on muscle repair and sought mechanisms supporting an injury/NSAID/infection axis. METHODS: A murine model of eccentric contraction (EC)-induced injury of the tibialis anterior muscle was used to profile the cellular and molecular changes induced by ketorolac tromethamine administered 47 hr post injury. RESULTS: NSAID administration inhibited several important muscle regeneration processes and down-regulated multiple cytoprotective proteins known to inhibit the intrinsic pathway of programmed cell death. These activities were associated with increased caspase activity in injured muscles but were independent of any NSAID effect on macrophage influx or phenotype switching. CONCLUSIONS: These findings provide new molecular evidence supporting the notion that NSAIDs have a direct negative influence on muscle repair after acute strain injury in mice and thus add to renewed concern about the safety and benefits of NSAIDS in both children and adults, in those with progressive loss of muscle mass such as the elderly or patients with cancer or AIDS, and those at risk of secondary infection after trauma or surgery.

A novel murine model of Clostridium sordellii myonecrosis: Insights into the pathogenesis of disease.

Clostridium sordellii infections have been reported in women following natural childbirth and spontaneous or medically-induced abortion, injection drug users and patients with trauma. Death is rapid and mortality ranges from 70 to 100%. Clinical features include an extreme leukemoid reaction, the absence of fever, and only minimal pain or erythema at the infected site. In the current study, we developed a murine model of C. sordellii soft tissue infection to elucidate the pathogenic mechanisms. Mice received 0.5, 1.0 or 2.0 × 10(6) CFU C. sordellii (ATCC 9714 type strain) in the right thigh muscle. All doses caused fatal infection characterized by intense swelling of the infected limb but no erythema or visible perfusion deficits. Survival rates and time to death were inoculum dose-dependent. Mice developed a granulocytic leukocytosis with left shift, the onset of which directly correlated with disease severity. Histopathology of infected tissue showed widespread edema, moderate muscle damage and minimal neutrophil infiltration. Circulating levels of granulocyte colony-stimulating factor (G-CSF), soluble tumor necrosis factor receptor I (sTNF-RI) and interlukin-6 (IL-6) were significantly increased in infected animals, while TNF-α, and IL-1ß levels were only mildly elevated, suggesting these host factors likely mediate the leukocytosis and innate immune dysfunction characteristic of this infection. Thus, this model mimics many of the salient features of this infection in humans and has allowed us to identify novel targets for intervention.

The roles of injury and nonsteroidal anti-inflammatory drugs in the development and outcomes of severe group A streptococcal soft tissue infections.

PURPOSE OF REVIEW: This review summarizes clinical and basic science evidence linking trauma and nonsteroidal anti-inflammatory drug (NSAID) use to initiation and progression of severe group A streptococcal (GAS) soft tissue infection. RECENT FINDINGS: New evidence includes recent clinical series and controlled studies that lend support to an NSAID/GAS association, basic science studies that demonstrate unique roles for nonpenetrating injury and NSAID administration in initiation of cryptogenic GAS infection and experimental studies showing that nonselective NSAIDs accelerate disease progression and limit antibiotic efficacy in established GAS soft tissue infections. Potential mechanisms for these processes are discussed. SUMMARY: NSAIDs are important anti-inflammatory and analgesic drugs; however, new experimental data suggest that nonselective NSAIDs do more than simply mask the signs and symptoms of developing GAS infection. A more thorough understanding of the triadic interplay of injury-triggered immune signaling, GAS soft tissue infection and NSAIDs is of significant clinical importance and could shift the current paradigm of pain management to avert the consequences of such devastating infections.

Cardiac myocyte dysfunction induced by streptolysin O is membrane pore and calcium dependent.

Septic cardiomyopathy is a severe complication among some patients who develop group A streptococcal toxic shock syndrome. Despite the importance of cardiac dysfunction in determining prognosis, very little is known about mechanisms that reduce cardiac output in association with streptococcal infection. Here, we investigated the effects of streptococcal extracellular toxins on mechanical contractility of electrically paced primary murine cardiomyocytes. Our data demonstrate that streptolysin O (SLO) is the major streptococcal toxin responsible for cardiomyocyte contractile dysfunction. Streptolysin O dose-dependently affected cardiac myocyte function in discrete stages. Exposure to SLO caused a failure of cardiac cells to respond to electrical pacing, followed by spontaneous dysregulated contractions and augmented strength of contraction. Central to these SLO-mediated effects is a marked influx of calcium into the cytosol through SLO-mediated pores in the cytoplasmic membrane. Such calcium mobilization in response to SLO correlated temporally with hypercontractility and unpaced contractions. During continued exposure to SLO, cardiomyocytes exhibited periods of reversion to normal electrical pacing suggestive of membrane lesion repair and restoration of calcium handling. Together, these observations are consistent with the clinical observation that septic cardiomyopathy is a reversible condition in patients who survive streptococcal toxic shock syndrome. These data provide strong evidence that streptococcal exotoxins, specifically SLO, can directly impact cardiac mechanical function.

Effects of selective and nonselective nonsteroidal anti-inflammatory drugs on antibiotic efficacy of experimental group A streptococcal myonecrosis.

BACKGROUND: Epidemiologic evidence suggests that nonsteroidal anti-inflammatory drugs (NSAIDs) contribute to more severe group A streptococcal (GAS) infections, yet a beneficial role for NSAIDs has been demonstrated in other experimental bacterial infections. METHODS: Nonselective (ketorolac tromethamine, ibuprofen, indomethacin), COX-1-selective (SC-560), or COX-2-selective (SC-236) NSAIDs ± antibiotics (penicillin, clindamycin) were given to mice challenged intramuscularly with M-type 3 GAS and disease course was followed for 14 days. RESULTS. All nonselective NSAIDs significantly accelerated mortality and reduced antibiotic efficacy; COX-selective NSAIDs had no significant effects. CONCLUSIONS: Use of nonselective NSAIDs, either alone or as adjuncts to antibiotic therapy, for GAS soft tissue infection may contribute to worse outcomes.

Pregnancy-related group a streptococcal infections: temporal relationships between bacterial acquisition, infection onset, clinical findings, and outcome.

Puerperal sepsis caused by group A Streptococcus (GAS) remains an important cause of maternal and infant mortality worldwide, including countries with modern antibiotic regimens, intensive care measures and infection control practices. To provide insights into the genesis of modern GAS puerperal sepsis, we reviewed the published cases and case series from 1974 to 2009, specifically seeking relationships between the likely source of pathogen acquisition, clinical signs, and symptoms at infection onset and patient outcomes that could provide clues for early diagnosis. Results suggest that the pathogenesis of pregnancy-related GAS infections in modern times is complex and not simply the result of exposure to GAS in the hospital setting. Additional research is needed to further explore the source of GAS, the specific M types involved, and the pathogenesis of these pregnancy-related infections to generate novel preventative and therapeutic strategies.

Staphylococcus aureus α-hemolysin promotes platelet-neutrophil aggregate formation.

Community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) causes severe hemorrhagic necrotizing pneumonia associated with high mortality. Exotoxins have been implicated in the pathogenesis of this infection; however, the cellular mechanisms responsible remain largely undefined. Because platelet-neutrophil aggregates (PNAs) can dysregulate inflammatory responses and contribute to tissue destruction, we investigated whether exotoxins from MRSA could stimulate formation of PNAs in human whole blood. Strong PNA formation was stimulated by toxins from stationary phase but not log phase CA-MRSA, and α-hemolysin was singularly identified as the mediator of this activity. MRSA exotoxins also caused neutrophil (polymorphonuclear leukocyte) activation, as measured by increased CD11b expression, although platelet binding was not driven by this mechanism; rather, α-hemolysin-induced PNA formation was solely platelet P-selectin dependent. These findings suggest a role for S. aureus α-hemolysin-induced PNA formation in alveolar capillary destruction in hemorrhagic/necrotizing pneumonia caused by CA-MRSA and offer novel targets for intervention.

Cardiac dysfunction in StrepTSS: group A streptococcus disrupts the directional cardiomyocyte-to-macrophage crosstalk that maintains macrophage quiescence.

Myocardial dysfunction in group A streptococcal (GAS) toxic shock syndrome (StrepTSS) is characterized by severe biventricular dilatation and a striking reduction in ventricular performance; however, the mechanisms have not been fully elucidated. We have previously shown that pro-inflammatory cytokines are upregulated in the hearts of experimental animals with GAS bacteremia and that cardiomyocytes themselves as well as macrophages are the principal cytokine sources. Although macrophage-derived cytokines can clearly affect cardiac contractility, we questioned whether soluble cardiomyocyte-derived mediators might in turn affect macrophage function. Thus, we sought evidence of cardiomyocyte-to-macrophage directional cross-talk under resting versus GAS-stimulated conditions, using production of matrix metalloproteinase-9 (MMP-9) as an indicator of such signaling. Our results demonstrate that unstimulated cardiomyocytes produce a soluble inhibitor/s that maintains macrophage functional quiescence. Further, viable GAS induced production of cardiomyocyte-derived stimulator/s that overcomes quiescence and boosts macrophages production of MMP-9 and expression of pro-inflammatory cytokines (IL-1ß, IL-6) and cardiodepressant factors (iNOS). Understanding the role of these cardiomyocyte-derived effectors of macrophage function (herein termed "cardiokines") in sepsis-associated cardiomyopathy may suggest new targets for therapeutic intervention.