INFECTIOUS DISEASES
1. Cover the middle and right-hand columns and specify which bugs are associated with
each type of infection and what type of empiric antibiotic should be used while waiting for
culture results.
CONDITION MAIN ORGANISM(S) EMPIRICAL ANTIBIOTICS
Urinary tract infection Escherichia coli Trimethoprim-sulfamethoxazole,
nitrofurantoin, amoxicillin,
quinolones
Bronchitis Virus, Haemophilus influenzae,Amoxicillin, erythromycin
Moraxella spp.
Pneumonia (classic) Streptococcus pneumoniae, Third-generation cephalosporin,
H. influenzae azithromycin
Pneumonia (atypical) Mycoplasma, Chlamydia spp. Macrolide antibiotic, doxycylcine
Osteomyelitis Staphyloccous aureus, Antistaphylococcal penicillin,!
Salmonella spp. vancomycin
Cellulitis Streptococci, staphylococci Antistaphylococcal penicillin (covers
both)
Meningitis (neonate) Streptococci B, E. coli, ListeriaAmpicillin + aminoglycoside, third-
spp. generation cephalosporin
Meningitis (child/adult)S. pneumoniae, Neisseria Third-generation cephalosporin or
meningitidis* meropenem + vancomycin +
dexamethasone
Sepsis Gram-negative organisms, Third-generation penicillin/
streptococci, cephalosporin staphylococci +
aminoglycoside, imipenem
Septic arthritis^ S. aureus Antistaphylococcal penicillin,
vancomycin
Gonococci Ceftriaxone, penicillin, spectinomycin
Endocarditis Staphylococci, streptococci Antistaphaylococcal penicillin (or
vancomycin) + aminoglycoside
*H. influenzae is no longer as common a cause of meningitis in children because of widespread vac-
cination. In a child with no history of immunization, H. influenzae is the most likely cause of menin-
gitis.
fExamples: dicloxacillin, methicillin.
$Think of staphylococci if the patient is monogamous or not sexually active. Think of gonorrhea for
younger adults who are sexually active.
2. Cover the right-hand columns and specify the empirical antibiotic of choice for each
organism.
ORGANISM* ANTIBIOTIC OTHER CHOICES
Strep A or B Pencillin, cephazolin Erythromycin
S. pneumoniae 3rd gen. cephalosporin, fluroquinolonFluoroquinolonee (e.g. levofloxacin)
Enterococcus Penicillin or ampicillin + Vancomycin + aminoglycoside
aminoglycoside
Staphylococcus aureus Anti-Staph Penicillin (e.g. methicillinVancomyci)n (MRSA)
Gonococcusf Ceftriaxone or fluoroquinolone Spectinomycin
118 Infectious Diseases
ORGANISM* ANTIBIOTIC OTHER CHOICES
Meningococcus Penicillin/ampicillin Cefotaxime, chloramphenicol
Haemophilus 2nd or 3rd gen. cephalosporin Ampicillin
Pseudomonas Antipseudomonal penicillin + Aztreonam, imipenem
aminoglycoside
Bacteroides Metronidazole Clindamycin
Mycoplasma Erythromycin, azithromycin Doxycycline
Treponema pallidum Penicillin Doxycycline
Chlamydia Doxycycline, azithromycin Erythromycin, fluoroquinolone
Lyme disease Ceftriaxone, doxycycline Erythromycin, amoxicillin
*Always use culture sensitivities to guide therapy once available
fWith genital infections, always treat for presumed Chalmydia co-infection with azithromycin or
doxycycline
3. Cover the right-hand column and specify what each Gram stain most likely represents.
GRAM STAIN RESULT MEANING
Blue/purple color Gram-positive organism
Red color Gram-negative organism
Gram-positive cocci in chains Streptococci
Gram-positive cocci in clusters Staphylococci
Gram-positive cocci in pairs Streptococcus pneumoniae
(diplococci)
Gram-negative coccobacilli Haemophilus sp.
(small rods)
Gram-negative diplococci Neisseria sp. (sexually transmitted disease, septic arthritis,
meningitis) or Momxella sp. (lungs, sinusitis)
Plump gram-negative rod with thick Klebsiells. sp.
capsule (mucoid appearance)
Gram-positive rods that form spores Clostridium sp., Bacillus sp.
Pseudohyphae Candida sp.
Acid-fast organisms Mycobacterium (usually M. tuberculosis), Nocardia sp.
Gram-positive with sulfur granules Actinomyces sp. (pelvic inflammatory disease in intra-
uterine device users; rare cause of neck mass/cervical
adenitis)
Silver-staining Pneumocystis carinii and cat-scratch disease
Positive India ink preparation Cryptococcus neoformans
(thick capsule)
Spirochete Treponema sp., Leptospira sp. (both seen only on dark-
field microscopy), Borrelia sp. (seen on regular light
microscope
4. What is the gold standard for diagnosis of pneumonia?
Sputum culture. Try to get the culture before starting antibiotics, though many treat empiri-
cally without culture in routine cases. Get blood cultures, too, because bacteremia is common
with pneumonia.
5. What is the most common cause of pneumonia? How does it classically present?
Streptococcus pneumoniae. Look for rapid onset of shaking chills after 1-2 days of upper
respiratory infection symptoms (sore throat, runny nose, dry cough), followed by fever, pleurisy,
and productive cough (yellowish-green or rust-colored from blood), especially in older adults.
Chest radiograph shows lobar consolidation, and the white blood cell count is high with a large
percentage of neutrophils. Treat with a third-generation cephalosporin (e.g., ceftriaxone),
macrolide (e.g., azithromycin), or broad-spectrum floroquinolone (e.g., levofloxacin).
Infectious Diseases 119
6. What is the best prevention against S. pneumoniael
Vaccination. Give pneumococcal vaccine to all children as well as adult patients over 65
years old, splenectomized patients, patients with sickle-cell disease (who have autosplenec-
tomy) or splenic dysfunction, immunocompromised patients (HIV, malignancy, organ trans-
plant), and all patients with chronic disease (e.g., diabetes, cardiac, pulmonary, renal, or liver
disease).
7. How do you recognize and treat Haemophilus influenzae pneumonia?
Probably the second most common cause of pneumonia, H. influenzae is more common in
children than adults. Often it resembles pneumococcal pneumonia clinically, but look for gram-
negative coccobacilli on sputum Gram stain. Treat with ampicillin/amoxicillin, cephalosporin, or
trimethoprim-sulfamethoxazole.
8. Describe the hallmarks of Staphylococcus aureus pneumonia.
S. aureus tends to cause hospital-acquired (nosocomial) pneumonia and pneumonia in
patients with cystic fibrosis (along with Pseudomonas sp.), intravenous drug abusers, and
patients with chronic granulomatous disease (look for recurrent lung abscesses). Empyema and
lung abscesses are relatively common with S. aureus pneumonia.
9. In what clinical situations do you tend to see gram-negative pneumonias?
Pseudomonas infection classically is associated with cystic fibrosis, Klebsiella infection with
"skid-row" alcoholics and homeless people (watch for classic description of currant jelly sputum),
and enteric gram-negative organisms (e.g., Escherichia coli) with aspiration, neutropenia, and hos-
pital-acquired pneumonia. These pneumonias often have a high mortality rate because of the type
of patients affected and the severity of the pneumonia (abscesses are common). Treat empirically
with a third-generation penicillin/cephalosporin plus an aminoglycoside.
10. How do you recognize Mycoplasma pneumonia?
Mycoplasma infection is most common in adolescents and young adults (the classic patient
is a college student or soldier who lives in a dormitory/barracks and has sick contacts). It is one
of the atypical pneumonias because it is different from pneumonia due to Streptococcus pneu-
moniae. For example, it has a long prodrome with gradual worsening of malaise, headaches, dry,
nonproductive cough, and sore throat; the fever tends to be low-grade. Chest radiograph shows a
patchy, diffuse bronchopneumonia and classically looks terrible, although the patient often does
not feel that bad. Look for positive cold-agglutinin antibody tilers, which may cause hemoly-
sis or anemia. Atypical pneumonia is treated empirically with a macrolide antibiotic
(azithromycin) or broad-spectrum fluoroquinolone (e.g., levofloxacin).
11. What about chlamydial pneumonia?
Chlamydia sp. is second only to Mycoplasma sp. as the cause of atypical pneumonia in ado-
lescents and young adults. It presents similarly but has negative cold-agglutinin antibody tilers.
Treat empirically with a macrolide antibiotic (azithromycin) or broad-spectrum fluoroquinolone
(e.g., levofloxacin).
12. In what setting do you see Pneumocystis carinii (PCP) and cytomegalovirus (CMV)
pneumonia?
In HIV-positive patients with CD4 counts < 200/mm 3 (AIDS) and other severely immuno-
suppressed patients (e.g., organ transplant recipients taking powerful immunosuppressants or
patients on cancer chemotherapy). In AIDS, PCP is the most common opportunistic pneumonia
and may require bronchoalveolar lavage for diagnosis. PCP can be seen with silver stains and typ-
ically causes bilateral interstitial lung infiltrates. Treat with trimethoprim-sulfamethoxazole; the
alternative is pentamidine. CMV pneumonia is characterized by intracellular inclusion bodies.
Treat with ganciclovir; foscarnet is an alternative.
