Bacteriology of the Marine Environment and Antibiotic Therapy

Ocean water provides a rich saline milieu for microbes.2 Although the greatest number and variety of bacteria are found near the ocean surface, diverse bacteria and fungi are found in marine silts, sediments, and sand. Microbes, including bacteria, microalgae, protozoa, yeasts, and viruses, are most abundant in areas with the greatest number of life forms. Marine bacteria are generally gram-negative rods. Bacteria isolated from the marine environment or from marine-acquired wounds of greatest concern to humans include Aeromonas hydrophila, Bacteroides fragilis, Chromobacterium violaceum, Clostridium perfringens, Erysipelothrix rhusopathiae, Escherichia coli, Mycobacterium marinum, Pseudomonas aeruginosa, Salmonella enteritidis, Staphylococcus aureus, Streptococcus species, and Vibrio species.

There is no substitute for meticulous wound care, including irrigation and débridement of devitalized tissue, with particular attention to retained foreign bodies, such as teeth, vegetable matter, and spines. Quantitative wound culture prior to the appearance of clinically evident wound infection has not been shown to be useful. The issue of prophylactic antibiotics in the treatment of marine wounds has not been well studied. Pending a prospective study of prophylactic antibiotics in this setting, the following recommendations are generally advised, based on the morbidity of soft-tissue infections caused by Vibrio species:3

• Minor abrasions and lacerations of the normal, immunocompetent patient do not require prophylactic antibiotics.

• Minor abrasions and lacerations of immunocompromised or chronically ill patients require initiation of therapy with trimethoprim-sulfamethoxazole, tetracycline, cefuroxime axetil, norfloxacin, or ciprofloxacin.

• High-risk wounds (e.g., extensive lacerations or burns; deep puncture wounds, particularly involving the joint space; or grossly contaminated wounds) require initiation of parenteral trimethoprim-sulfamethoxazole, a third-generation cephalosporin, an aminoglycoside, chloramphenicol, or one of the oral agents mentioned above.

The objectives for the management of infections from marine microorganisms are to recognize the clinical condition, culture the organism, and provide antimicrobial therapy. The appearance of an infection indicates the need for prompt débridement and search for a retained foreign body. Infected wounds should be cultured for aerobes and anaerobes. Since special media may be necessary for culture and sensitivity testing, the clinician should alert the microbiology laboratory that a marine-acquired organism might be present. Empirical antibiotic therapy should be initiated based on the clinical condition.

Management of marine-acquired infections must include coverage against Vibrio species with a third-generation cephalosporin, trimethoprim-sulfamethoxazole, tetracycline, norfloxacin, ciprofloxacin, or cefuroxime. Freshwater infections may be treated with the abovementioned agents, imipenem, or an aminoglycoside, to cover Aeromonas species. Appropriate antibiotic coverage against staphylococcal and streptococcal species is mandatory, since they remain the most common infecting organisms. Imipenem-cilastatin is reserved for established wound infections and/or sepsis.

Several special clinical conditions are of note. A patient with rapidly progressive cellulitis, myositis, or necrotizing fasciitis warrants consideration of Vibrio parahaemolyticus or Vibrio vulnificus infection. Vibrio vulnificus can also cause primary septicemia in chronically ill individuals, particularly those with hepatic disease. Mortality rates for such patients approach 60 percent. Erysipelothrix rhusopathiae is the infectious agent in "fish-handler's disease" and causes sharply marginated, painful, expanding plaques on the fingers or hands following cutaneous inoculation. It responds to penicillin, ampicillin, cephalexin, b-lactam antimicrobials, ciprofloxacin, and clindamycin. Resistance has been noted to vancomycin, the aminoglycosides, trimethoprim-sulfamethoxazole, tetracycline, and erythromycin. M. marinum is an acid-fast bacillus that causes "swimming-pool granuloma" or "aquarium granuloma." Some 3 to 4 weeks following an abrasion or puncture wound, the patient develops a red papule, which progresses to a cutaneous granuloma. Excision or antibiotics (minocycline, trimethoprim-sulfamethoxazole, rifampin, or ethambutol) are the treatments of choice, although spontaneous resolution typically occurs over 2 to 3 years. Aeromonas hydrophila is a gram-negative bacterium found in marine and freshwater environments; it causes wound infections that can rapidly become cellulitic and progress to necrotizing myositis.

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