Differential effects of myeloperoxidase-derived oxidants on Escherichia coli DNA replication

Oxidation of bacillithiol during killing of Staphylococcus aureus USA300 inside neutrophil phagosomes

Targeting immune evasion tactics of pathogenic bacteria may hold the key to treating recalcitrant bacterial infections. Staphylococcus aureus produces bacillithiol (BSH), its major low-molecular-weight thiol, which is thought to protect this opportunistic human pathogen against the bombardment of oxidants inside neutrophil phagosomes. Here, we show that BSH was oxidized when human neutrophils phagocytosed S. aureus, but provided limited protection to the bacteria. We used mass spectrometry to measure the oxidation of BSH upon exposure of S. aureus USA300 to either a bolus of hypochlorous acid (HOCl) or a flux generated by the neutrophil enzyme myeloperoxidase. Oxidation of BSH and loss of bacterial viability were strongly correlated (r = 0.99, p < 0.001). BSH was fully oxidized after exposure of S. aureus to lethal doses of HOCl. However, there was no relationship between the initial BSH levels and the dose of HOCl required for bacterial killing. In contrast to the HOCl systems, only 50% of total BSH was oxidized when neutrophils killed the majority of phagocytosed bacteria. Oxidation of BSH was decreased upon inhibition of myeloperoxidase, implicating HOCl in phagosomal BSH oxidation. A BSH-deficient S. aureus USA300 mutant was slightly more susceptible to treatment with either HOCl or ammonia chloramine, or to killing within neutrophil phagosomes. Collectively, our data show that myeloperoxidase-derived oxidants react with S. aureus inside neutrophil phagosomes, leading to partial BSH oxidation, and contribute to bacterial killing. However, BSH offers only limited protection against the neutrophil's multifaceted killing mechanisms.

Induction of the reactive chlorine-responsive transcription factor RclR in Escherichia coli following ingestion by neutrophils

Neutrophils generate hypochlorous acid (HOCl) and related reactive chlorine species as part of their defence against invading microorganisms. In isolation, bacteria respond to reactive chlorine species by upregulating responses that provide defence against oxidative challenge. Key questions are whether these responses are induced when bacteria are phagocytosed by neutrophils, and whether this provides them with a survival advantage. We investigated RclR, a transcriptional activator of the rclABC operon in Escherichia coli that has been shown to be specifically activated by reactive chlorine species. We first measured induction by individual reactive chlorine species, and showed that HOCl itself activates the response, as do chloramines (products of HOCl reacting with amines) provided they are cell permeable. Strong RclR activation was seen in E. coli following phagocytosis by neutrophils, beginning within 5 min and persisting for 40 min. RclR activation was suppressed by inhibitors of NOX2 and myeloperoxidase, providing strong evidence that it was due to HOCl production in the phagosome. RclR activation demonstrates that HOCl, or a derived chloramine, enters phagocytosed bacteria in sufficient amount to induce this response. Although RclR was induced in wild-type bacteria following phagocytosis, we detected no greater sensitivity to neutrophil killing of mutants lacking genes in the rclABC operon.

The effects of neutrophil-generated hypochlorous acid and other hypohalous acids on host and pathogens

Neutrophils are predominant immune cells that protect the human body against infections by deploying sophisticated antimicrobial strategies including phagocytosis of bacteria and neutrophil extracellular trap (NET) formation. Here, we provide an overview of the mechanisms by which neutrophils kill exogenous pathogens before we focus on one particular weapon in their arsenal: the generation of the oxidizing hypohalous acids HOCl, HOBr and HOSCN during the so-called oxidative burst by the enzyme myeloperoxidase. We look at the effects of these hypohalous acids on biological systems in general and proteins in particular and turn our attention to bacterial strategies to survive HOCl stress. HOCl is a strong inducer of protein aggregation, which bacteria can counteract by chaperone-like holdases that bind unfolding proteins without the need for energy in the form of ATP. These chaperones are activated by HOCl through thiol oxidation (Hsp33) or N-chlorination of basic amino acid side-chains (RidA and CnoX) and contribute to bacterial survival during HOCl stress. However, neutrophil-generated hypohalous acids also affect the host system. Recent studies have shown that plasma proteins act not only as sinks for HOCl, but get actively transformed into modulators of the cellular immune response through N-chlorination. N-chlorinated serum albumin can prevent aggregation of proteins, stimulate immune cells, and act as a pro-survival factor for immune cells in the presence of cytotoxic antigens. Finally, we take a look at the emerging role of HOCl as a potential signaling molecule, particularly its role in neutrophil extracellular trap formation.

Rapid degradation and non-selectivity of Dakin's solution prevents effectiveness in contaminated musculoskeletal wound models

BACKGROUND

Dakin's solution (buffered sodium hypochlorite) has been used as a topical adjunct for the treatment of invasive fungal infections in trauma patients. Prudent use of Dakin's solution (DS) for complex musculoskeletal wound management implies balancing antimicrobial efficacy and human tissue toxicity, but little empirical evidence exists to inform clinical practice. To identify potentially efficacious DS concentrations and application methods, we conducted two animal studies to evaluate the ability of DS to reduce bacterial burden in small and large animal models of contaminated musculoskeletal wounds.

METHODS

An established rat (Rattus norvegicus) contaminated femoral defect model was employed to evaluate the antimicrobial efficacy of DS as a topical adjunctive treatment for Staphylococcus aureus infection. A range of clinically-relevant DS concentrations (0.00025%-0.125%) were tested, both with and without periodic replenishment during treatment. Next, an established goat (Capra hircus) musculoskeletal wound model, consisting of a Pseudomonas aeruginosa contaminated proximal tibia cortical defect, muscle crush, and thermal injury, was utilized to evaluate the antimicrobial efficacy of dilute DS (0.0025% and 0.025%) as a surgical irrigant solution. In situ reactive chlorine concentrations were monitored throughout each treatment using an automated iodometric titration approach.

RESULTS

In a rat wound model, DS treatment did not significantly reduce S. aureus bioburden after 14 days as compared to saline control. Two treatment groups (0.01% single application and 0.025% multiple application) exhibited significantly higher bacterial burden than control. In a goat musculoskeletal wound model, neither 0.0025% nor 0.025% DS significantly altered P. aeruginosa bioburden immediately following treatment or at 48 h post-treatment. Overall, DS applied to exposed soft tissue exhibited rapid degradation, e.g., 0.125% DS degraded 32% after 5 s progressing to 86% degradation after 15 min following single application.

CONCLUSIONS

We did not observe evidence of a therapeutic benefit following Dakin's solution treatment for any tested concentration or application method in two contaminated musculoskeletal wound models. Despite confirmation of robust bactericidal activity in vitro, our findings suggest DS at current clinically-used concentrations does not kill tissue surface-attached bacteria, nor does it necessarily cause host tissue toxicity that exacerbates infection in the setting of complex musculoskeletal injury.

Redox reactions and microbial killing in the neutrophil phagosome

SIGNIFICANCE

When neutrophils kill microorganisms, they ingest them into phagosomes and bombard them with a burst of reactive oxygen species.

RECENT ADVANCES

This review focuses on what oxidants are produced and how they kill. The neutrophil NADPH oxidase is activated and shuttles electrons from NADPH in the cytoplasm to oxygen in the phagosomal lumen. Superoxide is generated in the narrow space between the ingested organism and the phagosomal membrane and kinetic modeling indicates that it reaches a concentration of around 20 μM. Degranulation leads to a very high protein concentration with up to millimolar myeloperoxidase (MPO). MPO has many substrates, but its main phagosomal reactions should be to dismutate superoxide and, provided adequate chloride, catalyze efficient conversion of hydrogen peroxide to hypochlorous acid (HOCl). Studies with specific probes have shown that HOCl is produced in the phagosome and reacts with ingested bacteria. The amount generated should be high enough to kill. However, much of the HOCl reacts with phagosomal proteins. Generation of chloramines may contribute to killing, but the full consequences of this are not yet clear.

CRITICAL ISSUES

Isolated neutrophils kill most of the ingested microorganisms rapidly by an MPO-dependent mechanism that is almost certainly due to HOCl. However, individuals with MPO deficiency rarely have problems with infection. A possible explanation is that HOCl provides a frontline response that kills most of the microorganisms, with survivors killed by nonoxidative processes. The latter may deal adequately with low-level infection but with high exposure, more efficient HOCl-dependent killing is required.

FUTURE DIRECTIONS

Better quantification of HOCl and other oxidants in the phagosome should clarify their roles in antimicrobial action.

Role of cellulose in protecting Shiga toxin-producing Escherichia coli against osmotic and chlorine stress

This study was undertaken to determine the role of cellulose in protecting Shiga toxin-producing Escherichia coli (STEC) against osmotic and chlorine treatments. STEC cells producing cellulose (19B and 49B) and their respective cellulose-deficient counterparts (19D or 49D) were subjected to osmotic (1, 2, and 3 M NaCl) or chlorine (25, 50, and 100 μg/ml sodium hypochlorite) treatments. The survival of STEC cells was determined at different treatment intervals. Populations of 19B cells were significantly higher (P < 0.05) than those of 19D cells at all sampling intervals for the chlorine treatments, at 24- to 48-h intervals for the 1 M NaCl treatment, and at 9- to 48-h intervals for the 2 M NaCl treatment. Significant differences in populations of 49B and 49D cells were observed after 9, 36, and 48 h of treatment with 2 M NaCl and after 3, 12, 36, and 48 h of treatment with 3 M NaCl (P < 0.05). Populations of 49B cells were higher than those of 49D cells (P < 0.05) also after 5 to 10 min of treatment with 50 μg/ml sodium hypochlorite and 3 to 10 min of treatment with 100 μg/ml sodium hypochlorite. The protective effects conferred by cellulose may explain the greater survival of cellulose-producing STEC under adverse environmental conditions.

Origin, structure, and biological activities of peroxidases in human saliva

Human whole saliva contains two peroxidases, salivary peroxidase (hSPO) and myeloperoxidase (hMPO), which are part of the innate host defence in oral cavity. Both hSPO as well as human milk lactoperoxidase (hLPO) are coded by the same gene, but to what extent the different producing glands, salivary and mammary glands, affect the final conformation of the enzymes is not known. In human saliva the major function of hSPO and hMPO is to catalyze the oxidation of thiocyanate (SCN(-)) in the presence of hydrogen peroxide (H(2)O(2)) resulting in end products of wide antimicrobial potential. In addition cytotoxic H(2)O(2) is degraded. Similar peroxidation reactions inactivate some mutagenic and carcinogenic compounds, which suggests another protective mechanism of peroxidases in human saliva. Although being target of an active antimicrobial research, the structure-function relationships of hSPO are poorly known. However, recently published method for recombinant hSPO production offers new tools for those investigations.