Bacteremia

Transient bacteremia may be caused by surgical manipulation of infected oral tissues or even routine dental manipulations; catheterization of an infected lower urinary tract; incision and drainage of an abscess; and colonization of indwelling devices, especially IV and intracardiac catheters, urethral catheters, and ostomy devices and tubes. Gram-negative bacteremia is typically intermittent and opportunistic; although it may have no effect on a healthy person, it may be seriously important in immunocompromised patients with debilitating underlying diseases, after chemotherapy, and in settings of malnutrition. The primary site of infection is usually in the lungs, in the GU or GI tract, or in soft tissues including the skin in patients with decubitus ulcer. It may also follow a dental procedure in patients at risk, and especially in patients with valvular heart disease, prosthetic heart valves, or other intravascular prostheses.

In chronically ill and immunocompromised patients, gram-negative bacteremia occurs more commonly. Additionally, these patients may develop bloodstream infections caused by aerobic bacilli, anaerobes, and fungi. Bacteroides may complicate abdominal and pelvic infections, especially when the female genital tract is infected.

Metastatic infection of the meninges or serous cavities, such as the pericardium or larger joints, can result from transient or sustained bacteremia. Endocarditis may occur (see Ch. 208), especially if the pathogen is an enterococcus, staphylococcus, or fungus, but is less common with gram-negative bacteremia. Staphylococcal bacteremia is common in IV drug users, and staphylococcus is a major cause of gram-positive bacterial endocarditis, including that involving the tricuspid valve.

Symptoms and Signs

Transient, low-level bacteremia is typically asymptomatic except in patients at special risk with sustained or higher-level bacteremia. The typical presentation includes systemic signs of infection, including tachypnea, shaking chills, a temperature spike, and GI symptoms (abdominal pain, nausea, vomiting, and diarrhea). These patients often present initially with warm skin and diminished mental alertness. Unless the BP is measured, hypotension may not be apparent. In some patients, the fall in BP is delayed.

Some features may help differentiate the cause and causative organism(s). Infections above the diaphragm are more likely to be due to gram-positive organisms. Infections in the abdomen, including the biliary and urinary tracts, are more likely to be due to gram-negative bacteria. However, no secure methods are available other than those based on laboratory diagnosis for differentiating gram-positive from gram-negative causes of bacteremia and septic shock.

Metastatic abscesses may occur almost anywhere and, when extensive, produce symptoms and signs characteristic of infection in the affected organ. Multiple abscess formation is especially common with staphylococcal bacteremia. Between 25 and 40% of patients with persistent bacteremia develop hemodynamic instability and therefore represent cases of septic shock.

Diagnosis

A Gram stain and culture should be performed on pus or fluid taken from all potential sites of infection, including infected body cavities, joint spaces, soft tissues, and skin lesions. Blood cultures should be performed for aerobic and anaerobic organisms. Two blood cultures, taken 1 h apart from two different phlebotomy sites, are sufficient for initial diagnosis of bacteremia. However, negative Gram stain or culture results do not exclude bacteremia, especially in patients who have had prior antibiotic therapy. This minimum of two blood culture specimens should be obtained from appropriately prepared phlebotomy sites. Cultures should also be performed on sputum and material from catheter insertion sites and wounds.

Formerly referred to as the sepsis syndrome, the systemic inflammatory response syndrome is defined by two or more of the following objective measurements: temperature > 38° C (> 100.4° F) or < 36° C (< 96.8° F); heart rate > 90 beats/min; respiratory rate > 20 breaths/min or Paco₂ < 32 mm Hg; WBC > 12,000 or < 4000 cells/µL, or > 10% immature forms. Typically, the WBC count is initially decreased to < 4,000/µL and then rises to > 15,000/µL with a marked increase in immature forms over an interval of between 2 and 6 h.

Prognosis and Treatment

Transient bacteremia associated with surgical procedures or with indwelling IV or urinary catheters is often undetected and requires no therapy except in patients with valvular heart disease, intravascular prostheses, or immunosuppression. In such patients, a prophylactic regimen of antibiotic therapy is advised, especially for prevention of endocarditis.

The outcome of more serious bacteremia depends on two predominant factors. The first is contingent on how quickly and thoroughly the source of infection can be eliminated. The second relates to the prognosis of the underlying disease and its accompanying systemic dysfunctions. Invasive devices, especially IV and urinary catheters, should be removed promptly. Antibiotic treatment should be started empirically after Gram stains and bacterial cultures have been obtained. In some cases (eg, rupture of a viscus, myometritis with abscesses, and gangrene of the intestine or gallbladder), surgery is mandatory. Large abscesses must be incised and drained and necrotic tissue removed. When persistent bacteremia is due to infection of the lungs, biliary tract, or urinary tract in the absence of obstruction or abscess formation, antibiotic therapy is usually successful. When multiple organisms are consistently recovered (polymicrobial bacteremia) in settings of multiorgan failure, a poor outcome is likely. Delay in antibiotic or surgical treatment markedly increases mortality.