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Methicillin-resistant Staphylococcus aureus (MRSA)
Evidence Levels: A Double-blind study B Clinical trial ≥ 20 subjects C Clinical trial < 20 subjects D Series ≥ 5 subjects E Anecdotal case reports
Methicillin-resistant Staphylococcus aureus (MRSA) bacteria have developed resistance to β-lactam antibiotics, such as methicillin, dicloxacillin, nafcillin, and oxacillin, and the cephalosporins, through horizontal gene transfer and natural selection. Although not necessarily more virulent than sensitive strains, infection caused by them is more difficult to treat due to their resistance to standard antibiotics.
Management Strategy
Healthcare-type strains of MRSA are associated with sepsis and are common colonists of chronic wounds, where debridement is typically the best strategy. Community-type MRSA infections typically present as an abscess or furunculosis. The most important intervention for an abscess remains surgical drainage . Purulent material can reform rapidly, so a punch or cruciate incision is advised, as it tends to remain open longer to allow adequate drainage. A curette is used to probe the wound and ensure that all pockets of purulent material have been adequately drained. Irrigation can be helpful, but packing is discouraged as purulent material often reforms behind the packing material. Evidence suggests that antibiotics can be beneficial even when an abscess is adequately drained.
Patients with significant surrounding cellulitis, refractory infection, or systemic manifestations require antibiotic therapy . Most community-associated (CA) MRSA strains are sensitive to trimethoprim-sulfamethoxazole. Most strains are also sensitive to doxycycline and minocycline , but doxycycline does not penetrate well into the nares. Clindamycin resistance is emerging in many communities. For serious infections, therapy should be guided by culture and sensitivity. Vancomycin, linezolid, dalbavancin, telavancin , and daptomycin are often effective. Rifampin improves intracellular killing of bacteria by vancomycin. Daptomycin may not penetrate reliably into pulmonary tissue, and tigecycline has a significant incidence of nausea. Ceftobiprole appears promising.
Spread occurs through skin-to-skin contact and via fomites. Decolonization may be indicated for prevention of recurrence or prevention of serious invasive disease in close contacts, but there is a high incidence of recurrence if the entire family unit is not treated. The most effective decolonization regimens address the nares and intertriginous and anogenital sites, as well as eczematous skin. In some patients, gut colonization may also be an issue. Widespread use of agents for decolonization is associated with emergence of resistance and alternative regimens are needed.
Specific Investigations
Cultures should be obtained from the contents of purulent infections and resistant infections. New molecular diagnostic tests are being developed to screen for MRSA and identify genes associated with resistance.
Role of nasal swab culture in guiding antimicrobial therapy for acute cellulitis in the era of community-acquired methicillin-resistant Staphylococcus aureus : a prospective study of 89 patients
Hsu MS, Liao CH, Fang CT. J Microbiol Immunol Infect 2019; 52: 494–7.
In a prospective study of 89 patients with acute cellulitis, nasal swab culture had high specificity (95%) in predicting MRSA cellulitis among the 24 patients whose cellulitis became purulent. Sensitivity is only 20%.
Cellulitis associated with an abscess or purulence is more likely to be staphylococcal. Most other cellulitis is streptococcal. Nasal culture may be of some value in predicting staphylococcal cases, but sensitivity is low.
Existence of multiple SCC mec elements in clinical isolates of methicillin-resistant Staphylococcus aureus
Nagasundaram N, Sistla S. J Med Microbiol 2019; 68: 720–7.
MRSA can be classified into hospital-acquired and community-associated strains based on SCC mec types. In this report, 118 of 200 isolates carried multiple SCC mec types and three were non-typable.
Community strains are now common in the hospital setting and hospital strains colonize wounds when patients are discharged. This report suggests that even lab typing shows considerable overlap.
Staphylococcus aureus and methicillin resistance detection directly from pediatric samples using PCR assays with differential cycle threshold values for corroboration of methicillin resistance
J Microbiol Methods 2019; 159: 167–73.
Two polymerase chain reaction (PCR) assays were evaluated: (1) a singleplex assay targeting the nucA gene; and (2) a multiplex PCR assay (mecA/SCC-orf PCRs) that detects the mecA gene and the conjunction region where SCC mec elements insert into the genome. The nucA PCR compared well with culture with an overall agreement of 93.1% and sensitivity and specificity of 93.5% and 93.0%, respectively.
Molecular tests are being used more frequently to replace conventional culture and sensitivity techniques.
Epidemiology
Interactions between atopic dermatitis and Staphylococcus aureus infection: clinical implications
Kim J, Kim BE, Ahn K, et al. Allergy Asthma Immunol Res 2019; 11: 593–603.
Multiple factors predispose to colonization of atopic skin including corneocyte adhesion, deficiency of antimicrobial peptides, microbial dysbiosis, decreased levels of filaggrin and filaggrin degradation products, overexpressed Th2/Th17 cytokines, and altered lipid profiles.
Strategies being evaluated to combat MRSA colonization include microbiome transplant, monoclonal antibodies against staphylococcal toxins, vaccines, and recombinant phage endolysin.
Multidrug and mupirocin resistance in environmental methicillin-resistant Staphylococcus aureus (MRSA) isolates from homes of people diagnosed with community-onset MRSA infection
Shahbazian JH, Hahn PD, Ludwig S, et al. Appl Environ Microbiol 2017; 31: 83.
MRSA was isolated from the household environment at the baseline and 3 months later. Patients and household members were randomized to receive mupirocin-based decolonization therapy or education. MRSA isolates were found in 68% (65/95) of homes at baseline and 51% (33/65) of homes 3 months later. The rates of multidrug resistance were 61% at baseline and 55% 3 months later. At the baseline, 100% (14/14) of MRSA isolates from rural homes demonstrated multidrug resistance.
Use of antimicrobial drugs, except clindamycin, was associated with multidrug resistance. Animal reservoirs are an important source of drug-resistant strains in rural settings.
Prognosis
Methicillin-resistant Staphylococcus aureus : an overview of basic and clinical research
Turner NA, Sharma-Kuinkel BK, Maskarinec SA, et al. Nat Rev Microbiol 2019; 17(4):203–18.
The epidemiology of MRSA is characterized by the serial emergence of epidemic strains with varying virulence. Although its incidence has recently declined in some regions, MRSA is still associated with high morbidity and mortality. MRSA is genetically diverse with varying virulence factors among different strains.
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