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CHAPTER 24 BACTERIAL PATHOGENS
Historically, Gram-negative bacilli have been the most commonly iso-
TREATMENT OF lated pathogens. These organisms include Klebsiella, Escherichia coli,
Pseudomonas, and Proteus. These bacteria are responsible for a variety
INFECTIONS IN THE of infections, including pneumonia, soft-tissue infections, perirectal
infections, and bacteremia. Urinary tract infections are less frequent
IMMUNOCOMPROMISED unless a urinary catheter is present or urinary tract obstruction has
developed. Meningitis is uncommon.
At present, roughly half of all documented infections in neutro-
HOST penic patients are caused by Gram-positive pathogens. This likely results
from the popularity of semipermanent venous catheters and from the
use of prophylactic regimens that are active against Gram-negative
rods. Staphylococcal species and Enterococcus are now the pathogens
Lisa Beutler and Jennifer Babik most frequently isolated from neutropenic patients. Several reports
2
document the increasing frequency of viridans group streptococci
as a major pathogen in neutropenic patients, especially in those receiv-
SUMMARY ing a hematopoietic stem cell transplant, perhaps because these patients
have a higher incidence of mucositis. Among infections caused by both
3
Infection is a major cause of morbidity and mortality in patients with severe Gram-negative and Gram-positive organisms, antibiotic resistance is a
inherited or acquired neutropenia or aplastic anemia, qualitative disorders of growing problem and is discussed under “Bacterial Infections” below.
neutrophils, and, notably, those persons receiving chemotherapy for treat- Anaerobic infections are less common unless periodontal or gastroin-
testinal pathology coexists.
ment of hematologic neoplasms. Severe neutropenia and monocytopenia Patients with Hodgkin lymphoma, other lymphomas, or chronic
often result from the combined effects of replacement of marrow with malig- lymphocytic leukemia primarily suffer from impaired cell-mediated
nant cells and superimposed intense chemotherapy. The severity and duration immunity and diminished antibody production. Consequently, the
4
of the neutropenia determine the risk of infection. Bacterial infections may spectrum of infections in these patients differs from that found in neu-
result in rapid clinical deterioration and even death. Fungal, viral, and parasitic tropenic patients. Bacterial infections, when they occur, tend to result
infections also may result in potentially lethal complications during or after from encapsulated organisms such as Pneumococcus or Haemophilus.
chemotherapy. This chapter considers methods of diagnosis of bacterial, fun- Listeria and Nocardia infections also are seen more frequently in this
gal, viral, and protozoal infection and describes treatment regimens. Because group of patients. 5
prevention of infection during periods of neutropenia should reduce morbidity
and improve outcome, attention is focused on prophylaxis against bacterial, FUNGAL PATHOGENS
parasitic, viral, and/or fungal infections. Fungal infections are common during periods of prolonged neutrope-
nia and in patients with lymphomas or chronic lymphocytic leukemia
who have impaired cell-mediated immunity. Candida species are most
frequently isolated. Historically, Candida albicans had been the most
common isolate; however, in recent years the number of non-albicans
RISK FACTORS AND INFECTING Candida infections has increased, partly as a consequence of wide-
6
ORGANISMS spread prophylaxis against C. albicans. The gastrointestinal tract serves
as a reservoir for Candida, and erosive esophagitis may develop. Can-
SEVERITY OF NEUTROPENIA dida may also enter the bloodstream via indwelling catheters.
Aspergillus and fungi that cause mucormycosis also may cause
Bacterial, fungal, viral, and parasitic organisms may cause infection in invasive disease. The use of mold-active prophylaxis may be associated
neutropenic patients. Bacterial infections are the most frequent and with an increased incidence of mucormycosis. These organisms tend to
1
7
usually the most serious. The risk for bacterial infection increases when colonize and infect the sinuses and bronchopulmonary tree.
the neutrophil count falls to less than 0.5 × 10 /L and becomes espe- Infections with Cryptococcus, Aspergillus, Coccidioides, Histo-
9
cially pronounced at neutrophil counts less than 0.1 × 10 /L. The rate of plasma, and Candida are more common in patients with leukemia or
1
9
decline and duration of neutropenia are important in determining the lymphoma who require chronic glucocorticoid treatment. Coccidioides
risk of bacterial infection. Disruption of mucosal barriers, especially in and Histoplasma are endemic mycoses. Coccidioides is endemic in
the oral cavity, esophagus, and bowel, further favors the development of the southwestern United States, in particular in Arizona and the San
infection by providing portals of entry. Joaquin Valley in California. Histoplasma is endemic in the Ohio and
Mississippi River Valleys. Emerging fungal infections with organisms
such as Scedosporium have become more common with increased use
of mold-active prophylaxis. 8
Pneumocystis jiroveci is a ubiquitous, endogenous fungus that may
Acronyms and Abbreviations: CMV, cytomegalovirus; CT, computed tomogra- cause pneumonia in neutropenic patients and in those with defective
phy; ESBL, extended-spectrum β-lactamase; Ig, immunoglobulin; IVIG, intravenous cell-mediated immunity.
immunoglobulin; LFT, liver function test; MRSA, methicillin-resistant Staphylococcus
aureus; PCP, Pneumocystis jiroveci pneumonia; RSV, respiratory syncytial virus; VRE, VIRAL PATHOGENS
vancomycin-resistant Enterococcus. Viral infections are especially frequent in patients with impaired
cell-mediated immunity. Among viruses that cause infections in

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