Microbiology - Gram-negative Bacteria

Summary Although Gram-positive bacteria, in particular Staphylococcus aureus and Streptococcus pyogenes are the most important causes of bacterial skin infections, many Gram-negative bacteria also cause serious skin infections. Pseudomonas aeruginosa is the most important of these, causing 10% of hospital acquired infections. It’s ubiquity, minimal nutrition requirements, biofilm formation, and antibiotic resistance make it problematic to control and treat. It is a particularly prominent cause of infection of burns, skin loss injuries and penetrating foot injuries. People spending long periods in poorly chlorinated pools or spas are prone to “hot tub folliculitis”. About 30% of people with P. aeruginosa bacteraemia get the serious skin disease, ecthyema gangrenosum. When a skin lesion is immersed in contaminated salty Vibrio vulnificus can cause deep life threatening infection, whereas in fresh waters Aeromonas hydrophila is a prominent cause of similar infections. Mycobacterium marinarum is often associated with people cleaning fish tanks, and causes “fish tank granulomas”. Respiratory secretions transmit Haemophilus influenzae and Mycobacterium leprae which cause the skin infections cellulitis and leprosy respectively. Neisseria meningitidis is also transmitted by respiratory secretions, and primarily causes meningitis and bacteraemia, often with a haemorrhagic rash manifestation in the skin. The bacteria Neisseria gonohorreae and Treponema pallidum cause the sexually transmitted diseases gonorrhoea and syphilis which result in urethritis and chancres, and as well may cause a disseminated rash on the body. There are many types of Gram-negative bacterial skin infections that are transmitted to humans from an animal reservoir. These include Bartonella henselae (cat scratch disease), Borrelia (lyme disease), Francisella tularensis (tularemia/ rabbit fever), Mycobacterium ulcerans (Buruli/Bairnsdale ulcers), Pasteurella multocida (cellulitis, lymphadenitis), and Rickettsia/Orientia which cause spotted fevers and typhus. With the exception of Bartonella henselae and Pasteurella multocida these Gram-negative skin infections from animal reservoirs are transmitted by ticks, mites, lice, fleas and insects.