BLOOD CULTURE GROWING A GRAM-POSITIVE ROD
A report from the laboratory that a blood culture contains a gram-positive rod suggests that the organism is a diphtheroid or Corynebacterium species, Clostridium species, Propionibacterium acnes, Listeria monocytogenes, or Bacillus species. Other possibilities, although they are much less common, include Rhodococcus equi, Erysipelothrix rhusiopathiae, Gardnerella vaginale, or Lactobacillus species. Some Corynebacterium species, P. acnes, and Bacillus species are part of the indigenous flora of the skin, and their isolation in a blood culture may represent nothing more than contamination. Other organisms, such as Listeria or Clostridium, may produce life-threatening disease, and their recovery from a blood culture may indicate the need for prompt therapy. A clinician who is assessing the importance of a blood culture positive for a gram-positive rod should consider the following factors:
Does the patient have a single positive blood culture or multiple sets of positive blood cultures obtained during several days? The finding of a single positive culture for a Corynebacterium species, P. acnes, or Bacillus species may represent nothing more than contamination. If the patient is febrile or has an implanted device, blood cultures should be repeated because, in selected clinical settings, these organisms can produce disease such as infective endocarditis. The patient with endocarditis will have multiple positive blood cultures because the bacteremia is generally continuous. In contrast, a single blood culture positive for an organism such as Listeria or, rarely, Rhodococcus or Erysipelothrix always indicates disease rather than contamination. Repeated blood cultures are indicated to determine if the patient has an illness associated with a sustained bacteremia. A single blood culture positive for a Clostridium species may be unimportant and secondary to a transient bacteremia, or it may reflect a life-threatening illness. Repeated blood cultures are indicated only if the patient is “ill.” Often, a blood culture positive for Clostridium becomes evident after the patient has been sent home from the hospital. In this situation, antimicrobial therapy is usually unnecessary.
Is the patient a normal or a compromised host? Although any of the gram-positive rods can infect a normal host, Listeria has a predilection to produce infection in patients with impaired cellular immunity. Pregnant women and neonates are also susceptible to Listeria. Rhodococcus is also an intracellular pathogen that has a predilection for infecting the compromised host. Corynebacterium jekeium usually causes infection in neutropenic patients with central venous catheters.
Does the patient have an implanted device, such as a cardiac valve prosthesis, arterial graft, ventriculoatrial shunt, or skeletal prosthesis? Although Staphylococcus epidermidis is a major pathogen in patients with implanted devices, rarely Corynebacterium species, P. acnes, or a Bacillus species can cause an infection in this setting. The finding of multiple positive blood cultures for one of these organisms during several days suggests an infection of a prosthetic device regardless of how innocuous the organism is considered. If a patient has a single positive blood culture for a grampositive rod and has an implanted device, obtain two to three sets of blood cultures within 24 hours. In a patient who is not receiving antimicrobials, this approach should detect the vast majority of cases of bacteremia. If the patient has an IV line and is febrile, the line should be changed and the tip cultured.
Corynebacterium species and P. acnes are the dominant organisms on the skin of adults. They are often called diphtheroids because they resemble, but do not include, Corynebacterium diphtheriae. Diphtheroids are aerobic gram-positive rods, do not form spores, and are not acid-fast. P. acnes organisms resemble diphtheroids and are anaerobic. The documentation of endocarditis caused by a diphtheroid may be difficult because the blood cultures may require prolonged incubation before demonstrating positivity, and Corynebacterium or Propionibacterium organisms frequently contaminate blood cultures. Propionibacterium is usually susceptible to penicillin, a cephalosporin, and the macrolides. Although it is an anaerobe, the organism is resistant to the imidazoles, such as metronidazole. Bacteremia caused by Corynebacterium species may be treated with a combination of penicillin with an aminoglycoside. If the patient is allergic to penicillin, vancomycin can be used.
Listeria is a b-hemolytic, aerobic, gram-positive rod that may be mistaken for a diphtheroid. In the laboratory, the organism has a characteristic tumbling motility. Both epidemic and sporadic disease occurs. Food-borne outbreaks were recognized in the 1980s, with coleslaw, milk, and cheese responsible for several outbreaks. Listeria has been isolated from uncooked beef, poultry, processed meats, and raw vegetables. It has been recommended that compromised hosts, pregnant women, and the elderly avoid soft cheeses, such as Mexican-style or feta cheese. Further, beef, pork, and poultry should be thoroughly cooked and raw vegetables well washed. Disease occurs mainly in pregnant women, neonates, and immunocompromised persons. Bacteremia and meningitis are the most common manifestations in nonpregnant persons. Focal infections such as endophthalmitis or a liver abscess are rare. In patients with meningitis, the cerebrospinal fluid glucose level is normal in more than 60% of patients, and the Gram’s stain shows organisms in fewer than 40% of cases. In patients with bacteremia, gastrointestinal symptoms such as nausea, vomiting, and diarrhea may be prominent.
Ampicillin plus an aminoglycoside such as gentamicin is the therapy of choice in a patient without a history of penicillin allergy. If the patient is allergic to penicillin, then trimethoprim-sulfamethoxazole (TMP-SMX) or vancomycin can be selected. Cephalosporins should be avoided. Although controlled studies are lacking, patients with meningitis should be treated for 3 weeks to prevent relapse, and those with endocarditis should be treated for 4 weeks. The management of patients with infected synthetic grafts should consist of surgical resection in combination with 6 weeks of antimicrobial therapy.
Most Bacillus species isolated in the laboratory, except for Bacillus anthracis, are contaminants. The organism is an aerobic, gram-positive rod that forms spores. Bacillus cereus and Bacillus subtilis have been associated with post-traumatic endophthalmitis. Bacteremia generally occurs in the compromised patient with a central venous line. The IV catheter should be removed, and vancomycin plus an aminoglycoside should be administered. The organism is often resistant to penicillin and other b-lactam drugs, including the new cephalosporins. Clindamycin is an alternative drug in patients unable to tolerate vancomycin.
R. equi is a recently recognized human pathogen that can cause cavitary pulmonary disease and bacteremia in the compromised host. Although the optimal therapy is unknown, the organism is susceptible to vancomycin, erythromycin, clindamycin, or TMP-SMX. Most authors favor selecting two agents, with possible surgical drainage for an abscess. Prolonged therapy is usually required.
Clostridium species are anaerobic rods that are usually gram-positive. In clinical specimens, the organisms may appear to stain as gram-negative bacilli. Spore formation occurs but may not be present in clinical specimens. Clostridia are an interesting group of organisms in that a positive blood culture has been associated with life-threatening disease or an “insignificant transient bacteremia.” Clostridium perfringens accounts for about 60% of blood culture isolates. An intraabdominal focus should be searched for in an ill patient with clostridial bacteremia. Some patients will have colon cancer. Clostridium septicum bacteremia is unusual; when present, it is often associated with an occult colon malignancy (about 40%). C. septicum bacteremia also occurs in patients with leukemia or in diabetic patients with infected foot ulcers. C. perfringens and C. septicum usually respond to penicillin or clindamycin. Approximately 25% of strains of C. perfringens are resistant to metronidazole. Rarely, a patient will have a blood culture positive for Clostridium tertium. Infection with C. tertium usually has a gastrointestinal source and often will respond without surgery to vancomycin or TMP-SMX. Interestingly, C. tertium is often resistant to clindamycin, penicillin, and metronidazole. (N.M.G.)
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Usually causes bacteremia in the presence of a foreign body. Organism is commonly susceptible to penicillin and resistant to metronidazole.
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Isolation of Eubacterium or Lactobacillus species from blood usually indicates contamination, but these organisms may rarely cause serious illness.
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A rare cause of endocarditis, which responded to penicillin.
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Cure is unlikely without removal of the catheter.
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Gastroenteritis caused by contaminated milk.
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R. equi is a rare cause of pulmonary infiltrates; infection is usually diagnosed by a positive blood culture (83%).
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A cause of a slowly progressive cavitary pneumonia.
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Comprehensive review of the various Coryneform bacteria.
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Clostridial bacteremia may be clinically unimportant.
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Ampicillin plus gentamicin remains the therapy of choice because the combination is bactericidal. TMP-SMX is an alternative in the penicillin-allergic patient.
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Lactobacilli occur as part of the normal gastrointestinal and genitourinary flora. Bacteremia is often (60%) polymicrobial in patients with underlying illnesses.
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Classic review. The majority of patients had endocarditis.
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Classic review. Penicillin is usually effective.
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In a patient with a soft-tissue infection caused by C. septicum in the absence of trauma, search for a gastrointestinal malignancy.
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An uncommon pulmonary pathogen in the compromised host.
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The incidence of anaerobic bacteremia appears to be decreasing.
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Penicillin plus an aminoglycoside should be effective.
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Most patients had a gastrointestinal source, either a colonic neoplasm or mucosal translocation.
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A rare cause of bacteremia in fishermen and meat handlers.
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A pathogen in compromised hosts. Penicillin is the drug of choice.
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Listeria was isolated from food in the refrigerators in 64% of infected patients. Ready-to-eat foods often grew serotype 4b, a disease-producing strain.
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Lactobacilli are found in dairy products and colonize the gastrointestinal tract. Despite their widespread occurrence, bacteremia is rare.
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Patients were likely to have eaten soft cheese, sliced meats, cheese from a store delicatessen, or poultry products.
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Underlying conditions included IV drug use, intravascular catheters, and malignancy.
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An uncommon cause of bacteremia in patients with a hematologic malignancy and granulocytopenia.
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An unusual cause of bacteremia.
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TMP-SMX is an alternative drug in the penicillin-allergic patient.