Minor wound infections may simply require local drainage and no microbiological testing. Wound swab or MCS pus (microscopy and culture) for moderate or severe infection or where spreading cellulitis or evidence of systemic infection. From a microbiological point of view, aspiration of pus is preferable to a swab of pus or wound. Be alert to skin infections with unusual exposures that may call for antibiotics against “unusual” organisms.
Trauma, including surgery:
- Staphylococcus aureus
- Streptococcus pyogenes
- Bacteroides fragilis
With soil (incl. faecal) contamination or thorn injury:
- atypical mycobacteria
- Nocardia
- fungi
- Sporothrix schenckii
- Clostridium perfringens
- Clostridium tetani – diagnosis based on clinical features but consider in any wound, especially penetrating or contaminated: send tissue from wound excision or debridement for microscopy and culture for Clostridium tetani, although the organism is rarely isolated
- farmers
- IV drug users (also MRSA, GABHS)
- burns
- umbilical cord infection (neonatal tetanus)
Tetanus is caused by the Clostridium tetani bacterium, commonly found in the environment within soil, dust, and manure and, where contaminate wounds, they produce the toxin which causes the symptoms of painful muscular contractions and spasms. Unvaccinated people remain at risk of developing tetanus if a wound or cut becomes contaminated.
With water contamination:
- Aeromonas hydrophila (freshwater)
- Vibrio spp. (brackish water)
- Chromobacterium violaceum
- Shewanella species
- Burkholderia sp.
- Erysipelothrix rhusiopathiae (fish handling)
- Mycobacterium marinum (fish-tank exposure)
Hot tub exposure:
- Pseudomonas aeruginosa
- atypical mycobacteria
Cat bite:
Pasteurella multocida – Pasteurella species are zoonotic pathogens, and humans can acquire an infection from domestic pet bites. Pasteurella occurs in many cats’ mouths, a large percentage of dog mouths, and frequently in rabbits. Wound swab, Blood culture, Joint aspirate.
Dog bite:
- Capnocytophaga canimorsus (dogs)
- Pasteurella canis
Human bite:
- Eikenella corrodens
- Viridans group Streptococci
Rat bite:
- Streptobacillus moniliformis
Immunocompromised or neutropaenic host:
- Pseudomonas aeruginosa
- Cryptococcus spp.
- Nocardia
- mycobacteria
Studies involving a detailed microbiological analysis of the aerobic and anaerobic microbiology of infected wounds:
| Reference | Study description and no. of wounds | No. of microbial isolates (% that were anaerobes) | Predominant isolates |
| Sanderson et al. 1979 | Anaerobes in 65 purulent post-appendectomy wounds (swab samples) | 179 (54) | E. coli, Bacteroides spp., Peptostreptococcus spp. |
| Brook et al. 1981 | Analysis of 209 cutaneous abscesses in children | 467 (58) | S. aureus, Streptococcus spp., E. coli, Bacteroides spp. |
| Wheat 1986 | Analysis of 131 infected diabetic foot ulcers in 130 patients | 538 (21) | Peptostreptococcus spp., Enterococcus spp., Staphylococcus spp. |
| Brook 1989 | 89 specimens from postsurgical abdominal wound infections | 235 (55) | E. coli, Bacteroides spp., Peptostreptococcus spp., Clostridium spp. |
| Brook 1989 | Specimens from 74 patients with post-thoracotomy sternal wound infections | 87 (22) | S. epidermidis, S. aureus, coliforms, Peptostreptococcus spp. |
| Brook 1989 | Analysis of pus from a Bartholin’s abscess in 28 patients | 67 (64) | Bacteroides spp., Peptostreptococcus spp., E. coli |
| Brook et al. 1990 | Analysis of 676 cutaneous abscesses | 1,702 (65) | Bacteroides spp., Peptostreptococcus spp., S. aureus, Clostridium spp., Fusobacterium spp. |
| Johnson et al. 1995 | Swab samples from 43 diabetic foot ulcers (46 infected sites) | 285 (36) | Peptostreptococcus spp., Prevotella spp., Bacteroides spp. (emphasis on anaerobes) |
| Brook 1995 | Analysis of pus from gastrostomy site wound infections in 22 children | 102 (44) | E. coli, Peptostreptococcus spp., Enterococcus spp., Bacteroides spp., S. aureus |
| Summanen et al. 1995 | Comparison of the microbiology of soft tissue infections in IVDUa and non-IVDU (160 abscesses sampled) | 304 (43) from IVDU; 222 (48) from non-IVDU | S. aureus, “Streptococcus milleri,” Peptostreptococcus spp., Prevotella spp., Bacteroides spp., Streptococcus pyogenes |
| Brook 1996 | Microbiology of specimens from 8 children with necrotizing fasciitis | 21 (62) | Peptostreptococcus spp., Streptococcus pyogenes, Bacteroides spp. |
| Di Rosa et al. 1996 | Role of anaerobes in 300 postoperative wound infections | 639 (23) | Clostridium spp., Bacteroides spp., Peptostreptococcus spp. (emphasis on anaerobes) |
| Mousa 1997 | Swab samples of burn wounds from 127 patient | 377 (31) | P. aeruginosa, S. aureus, Bacteroides spp., Peptostreptococcus spp., Klebsiella spp. |
| Brook et al. 1997 | Analysis of perirectal abscesses in 44 patients | 456 (72) | B. fragilis group, Peptostreptococcus spp., Prevotella spp., S. aureus, Streptococcus spp. |
| Brook et al. 1998 (45) | Analysis of 368 specimens from 340 trauma patients with wound infection | 711 (63) | B. fragilis group, Peptostreptococcus spp., Clostridium spp., S. aureus, Prevotella spp. |
| Brook 1998 | Analysis of 175 specimens from 166 children with infected traumatic wounds | 521 (70) | Peptostreptococcus spp., Prevotella spp. Fusobacterium spp., S. aureus, B. fragilis group |
| Pathare et al. 1998 | Pus or tissue specimens from 252 diabetic foot infections | 775 isolates (29) | Staphylococcus spp., Streptococcus spp., Peptostreptococcus spp. |
| Bowler et al. 1999 | Swab samples of 44 infected leg ulcers (based on clinical signs) | 220 isolates (49) | Peptostreptococcus spp., coliforms, coagulase-negative staphylococci, pigmented and nonpigmented gram-negative bacteria (anaerobes), fecal streptococci |
Risk Factors for MRSA (Bystritsky R. and Chambers H., Annals of Internal Medicine, 2018.):
- History of MRSA infection
- Nasal inhalation or smoking of illegal drugs or injection drug use
- Recent incarceration
- Contact sports
- Frequent visits to bars, raves, or clubs
- HIV infection
- Recent antibiotic use
- Recent hospitalization
- Haemodialysis
- Close contact with known or suspected MRSA infection
| Definitions of Skin and Soft Tissue Infections | |
| Cellulitis | Acute infection of skin involving deep dermis and subcutaneous fat |
| Erysipelas | More superficial infection of the skin, involving the lymphatics; characterized by a tender, erythematous plaque with well-demarcated borders |
| Folliculitis | Superficial infection of the hair follicle with purulence in the epidermis |
| Furuncle | Infection of the hair follicle with associated small subcutaneous abscess |
| Carbuncle | A cluster of furuncles |
| Cutaneous abscess | Localized collection of pus within the dermis and deeper skin tissues |
| Pyomyositis | Purulent infection of skeletal muscle, often with abscess formation |
| Impetigo | Superficial infection of the skin characterized by pustules or vesicles that progress to crusting or bullae |
| Ecthyma | A deeper variant of impetigo; begins as vesicles/pustules and evolves into “punched-out”–appearing ulcers |
| Gas gangrene | Necrotizing infection involving muscle; also known as clostridial myonecrosis |
| Necrotizing fasciitis | Aggressive infection of the subcutaneous tissue that spreads along fascial planes |
Cellulitis
Pus or aspirate from edge of lesion – Wound swab microscopy and culture. Blood culture if indicated. Requiring admission – Consider: Blood culture, Urea, Electrolytes, Creatinine, Glucose, Liver function tests, Full blood count, Ward test urine.
Common pathogens:
- Streptococcus pyogenes – Antistreptolysin O titre, Anti-deoxyribonuclease B Ab.
- Staphylococcus aureus
- Clostridium perfringens
Unusual pathogens:
- Vibrio sp. (vulnificus, parahaemolyticus) – Predisposing disorders include Cirrhosis, Diabetes mellitus, and tropical marine environments.
- Aeromonas hydrophila
- Shewanella sp.
- Chromobacterium violaceum
Animal bites:
- Pasteurella multocida
- Capnocytophaga canimorsus
Human bites:
- Eikenella corrodens
| ANTIMICROBIALS COMMONLY USED FOR ORAL THERAPY FOR CELLULITIS AND SOFT TISSUE INFECTIONS | ||
|---|---|---|
| Agent | Usual Dosage* | Comments |
| Streptococci | ||
| Amoxicillin | 500 mg PO TID | Useful for Pasteurella species, better bioavailability than penicillin |
| Penicillin VK | 500 mg PO QID | Narrow spectrum; frequent dosing |
| Streptococci and MSSA | ||
| Amoxicillin-clavulanate | 875/125 mg PO BID | Includes anaerobic coverage |
| Dicloxacillin | 500 mg PO QID | Frequent dosing |
| Cephalexin | 500 mg PO QID | Frequent dosing |
| MRSA (and streptococci/MSSA) | ||
| Clindamycin | 300 mg PO BID | Greatest association with secondary Clostridium difficile infection |
| Doxycycline | 100 mg PO BID | Causes photosensitivity; fewer clinical data |
| Trimethoprim-sulfamethoxazole | 1 double-strength tablet PO BID | Can cause hyperkalemia; use caution in patients with impaired renal function or on angiotensin-converting enzyme inhibitors/angiotensin-receptor blockers |
| Linezolid | 600 mg PO BID | Risk for serotonin syndrome with concomitant selective serotonin reuptake inhibitors; bone marrow toxicity with prolonged use |
| Tedizolid | 200 mg PO BID | Expensive; less risk for thrombocytopenia and drug interactions than linezolid |
| Delafloxacin | 450 mg PO BID | Limited clinical experience |
| BID = twice daily; MRSA = methicillin-resistant Staphylococcus aureus; MSSA = methicillin-sensitive Staphylococcus aureus; PO = oral; QID = 4 times daily; TID = 3 times daily. * May vary on the basis of weight, renal function, and indication. |
Prophylactic antibiotics can prevent recurrence in patients with frequent episodes of cellulitis and may be considered for patients who have 3 to 4 episodes per year despite attempts to treat or control predisposing factors. Regimens that include oral penicillin, intramuscular penicillin, and erythromycin have been investigated.[1]
Beyond irrigation and debridement, primary closure of uncomplicated dog-bite, affording improved cosmesis, can be considered, according to a 2014 randomized trial showing similar infection rates to those wounds allowed to heal by secondary intention (Paschos et al., Injury 45; (2014), 237-40.).
In addition to antibiotic prophylaxis, tetanus vaccination should be considered if immunizations are not up-to-date. The need for rabies post-exposure prophylaxis should also be assessed.
Non-infectious differentials:
- stasis dermatitis
- superficial thrombophlebitis
- deep venous thrombosis
- congestive cardiac failure
- drug reactions
- insect bites
- cutaneous vasculitis
- acute gout
Necrotising fasciitis:
- Type 1 (polymicrobial) – mixed anaerobes and aerobes: Streptococci, Clostridium species, Bacteroides species, Enterobacteriaceae, staphylococci, enterococci
- Type 2 (monomicrobial)
- Group A streptococci (most common)
- community-associated MRSA
- Clostridium species (infrequent)
- Vibrio species
References
Bowler, P G et al. “Wound microbiology and associated approaches to wound management.” Clinical microbiology reviews vol. 14,2 (2001): 244-69. doi:10.1128/CMR.14.2.244-269.2001.
Bystritsky, Rachel and Chambers, Henry. “Cellulitis and Soft Tissue Infections.” Annals of Internal Medicine (2018), ITC19. Annals of Internal Medicine 2018. doi:10.7326/AITC201802060.
Paschos NK, Makris EA, Gantsos A, Georgoulis AD. Primary closure versus non-closure of dog bite wounds. a randomised controlled trial. Injury. 2014;45:237-40. [PMID: 23916901]
RCPA Manual. “Wound infection.” Royal Australasian College of Pathologists. Oct 17, 2014. Available at http://www.rcpa.edu.au/Library/Practising-Pathology/RCPA-Manual/Items/Clinical-Problems/W/Wound-infection.
