Treating Community-acquired Bacterial Respiratory Tract Infections: Update on Etiology, Diagnosis, and Antimicrobial Therapy

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Treating Community-acquired Bacterial Respiratory Tract Infections: Update on Etiology, Diagnosis, and Antimicrobial Therapy

J Fam Pract. 2005 April;54(4):357-364

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References

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TABLE 2

Treatment of acute exacerbations of chronic bronchitis⁸

Type	Symptoms*	Antimicrobial therapy
Type 1 — Severe exacerbation	3 of 3 symptoms	More benefit than when treated with placebo
Type 2 — Moderate exacerbation	2 of 3 symptoms	Less benefit than when treated with placebo
Type 3 — Mild exacerbation	1 of 3 symptoms	No benefit
* Increased dyspnea, increased sputum volume, and increased sputum purulence.

Community-acquired pneumonia

Patients with CAP usually present with acute onset of lower respiratory symptoms (TABLE 1). Older and immunosuppressed patients may present with nonrespiratory symptoms, such as confusion, worsening of a chronic condition, or even falls. It is important to note that no combination of clinical signs and symptoms is a definitive diagnosis of CAP.¹⁹ All patients with suspected CAP should have a chest radiograph with posteroanterior and lateral views, as radiographs are essential for confirming a diagnosis.³ The Infectious Diseases Society of America (IDSA) and the American Thoracic Society support use of the Pneumonia PORT (Pneumonia Outcomes Research Team) Severity Index (PSI) as a means of risk stratification, combined with careful assessment of the patient and use of clinical judgment when determining whether a patient can be treated on an outpatient basis or should be hospitalized.¹⁰

Although it is important to attempt to identify the infecting organism, antimicrobial treatment for CAP is empiric because of the time it takes to get laboratory results and the potential for rapid deterioration of the patient’s condition. Further, patients with CAP may not produce sputum for Gram stain and culture; or, if they do, it may be mixed with upper respiratory tract secretions. Streptococcus pneumoniae may not grow from sputum culture; or, if found, it may be unclear whether the patient is colonized or infected.²⁰ Bacteria cultures may not be helpful either, as blood cultures usually are sterile in patients with CAP.²⁰

Selection of antimicrobial therapy

Once a bacterial infection is suspected, antibiotic treatment should be initiated promptly. Antibiotic therapy for ABS, ABECB, and CAP is simplified somewhat because the distributions of bacterial pathogens associated with each infection overlap substantially. The pathogens encountered most often in patients with ABS are Spneumoniae, Haemophilusinfluenzae, and Moraxella catarrhalis. To a lesser extent, anaerobic bacteria, other streptococcal species (including S pyogenes, S intermedius, and ahemolytic streptococci), and Staphylococcus aureusalso have been shown to cause ABS.²¹

Bacteria are isolated from the sputum of 40% to 60% of patients with AECB, and the predominant species are consistent with those listed for ABS. Atypical respiratory pathogens, most notably Chlamydophila(previously Chlamydia) pneumoniae, account for about 5% to 10% of organisms isolated from patients with ABECB.¹⁷ Patients with CAP also tend to be infected with the above-listed typical pathogens as well as the atypical Mycoplasma pneumoniae, C pneumoniae, and Legionella pneumophila. TABLE 3 lists pathogens associated with community-acquired bacterial RTIs and the effectiveness of commonly used antibiotics and telithromycin, a recently approved antimicrobial agent.

TABLE 3

Pathogens associated with ABS, CAP, and ABECB and in vitro effectiveness of commonly used antibiotics and telithromycin⁴⁰

	Amoxicillin	Amoxicillin/Clavulanate	Cefuroxime	Erythro-mycin	Clarithro-mycin	Azithro-mycin	Moxi-floxacin	Telithro-mycin
Streptococcus pneumoniae*	+	+	+	+	+	+	+	+
Resistant	±^†	–	–	–	–	–	+	+
Haemophilus influenzae
β-lactamase negative	+	+	+	±	±	±	+	+
β-lactamase positive	–	+	+	±	±	±	+	+
Moraxella catarrhalis	+	+	+	+	+	+	+	+
Mycoplasma pneumoniae	–	–	–	+	+	+	+	+
Chlamydophila pneumonoae	–	–	–	+	+	+	+	+
Legionellasp	–	–	–	+	+	+	+	+
ABS = acute bacterial sinusitis
CAP = community-acquired pneumonia
ABECB = acute bacterial exacerbations of chronic bronchitis
+ = effective.
– = not active.
±= significant resistance, but active against most strains.
*Resistance is increasing; nationwide survey suggests 21% to 43% resistance to penicillin.
†Amoxicillin doses of 80 mg/kg/d may be effective against nonmeningeal, penicillin-resistant S pneumoniae.

Resistance of bacterial pathogens

Although the common causative pathogens for community-acquired bacterial RTIs are few, selection of antibiotic therapy is becoming more complicated by the increasing rate of bacterial resistance to many of the antibiotics commonly used to treat these RTIs.

Resistance has been documented for all organisms associated with community-acquired RTIs. Current estimates indicate that 25% to more than 50% of Spneumoniaestrains are not completely susceptible to penicillin and that nearly one third of strains may be resistant to macrolides.²² Penicillin-resistant Spneumoniaealso may have reduced susceptibility to other antibiotics, including tetracycline, erythromycin, azithromycin, cephalosporins, clindamycin, trimethoprim/sulfamethoxazole (TMP/ SMX), and chloramphenicol.²³Streptococcuspneumoniae have developed resistance to fluoroquinolones as well. This is of particular concern to the Centers for Disease Control and Prevention (CDC), having influenced its recommendations regarding empiric treatment of pneumonia.¹⁰ Macrolides have been mainstays in empiric therapy of CAP because of their activity against both typical and atypical respiratory pathogens; however, their effectiveness has been compromised by a rapid rise in resistance by S pneumoniae. Recent evaluation of Spneumoniae isolates collected between 2000 and 2001 showed that resistance rates to erythromycin, clarithromycin, and azithromycin had increased to 31.0%, 30.7%, and 31.0%, respectively.²⁴ Further, results from 1 recent surveillance study that examined macrolide resistance among patients with community-acquired RTIs being treated by primary care physicians indicated that 23% to 33% of Spneumoniaeisolates were resistant to macrolides.^24,25 Resistance also is becoming prevalent among other pathogens associated with community-acquired RTIs, including H influenzaeand Mcatarrhalis.^25-27 This may complicate management of patients with community-acquired RTIs; infection with a treatment-resistant pathogen can increase risk for morbidity and mortality if treatment fails to eradicate the bacteria.^20,28 However, the clinical impact of resistance remains controversial. Resistance may be overcome with the use of an antibiotic with pharmacokinetic properties or a dosing regimen that achieves very high drug concentrations at sites of infection. Host defense mechanisms also may contribute to eradication of organisms with in vitro resistance to an antibiotic.^29,30 Further, since many infections are caused by viruses with high-resolution rates and since some bacterial infections resolve due to host response, the in vivo efficacy of an antibiotic may be much higher than its in vitro sensitivity.