6.3 Sterile Body Fluids: Pathogens, Flora, and Molecular Methods

Sources That the Outline Treats as Normally Sterile

Outline II.C covers body fluids from normally sterile sites. The named sources you should be able to process and interpret are pleural, peritoneal, pericardial, synovial, amniotic, and ocular (vitreous and aqueous humor). Each arrives as a tap, drain, or surgical collection. Each deserves a Gram stain, inoculation to enriched aerobic media (BAP, CHOC, and often MacConkey), and an enrichment broth. Add anaerobic media when the fluid can reflect bowel perforation, deep abscess, or bite-adjacent joints—not as a decorative extra on every clear synovial fluid from a crystal-arthritis workup, but as a requirement for peritoneal and abscess-associated specimens.

Pleural fluid (thoracentesis, chest-tube, empyema drainage) follows pneumonia and parapneumonic effusion: S. pneumoniae, S. aureus, beta-hemolytic streptococci, H. influenzae, and, in healthcare lungs, P. aeruginosa and Enterobacteriaceae. Frank empyema is often polymicrobial once oral anaerobes join. Pericardial fluid is less common; S. aureus, streptococci, and contiguous pneumonia organisms lead, and the volume is often tiny, so stain plus CHOC/BAP/broth come first. Synovial fluid (arthrocentesis) is the septic-arthritis specimen: S. aureus in all ages, S. pneumoniae and H. influenzae in children, Neisseria gonorrhoeae in sexually active adults (chocolate, CO2, never refrigerate), GBS and Gram-negative rods in older and diabetic patients. Amniotic fluid and intra-amniotic infection grow GBS, E. coli, other Enterobacteriaceae, anaerobes, and occasionally L. monocytogenes. Vitreous and aqueous humor from endophthalmitis may grow S. aureus, coagulase-negative staphylococci (post-cataract), streptococci, or Gram-negative rods including P. aeruginosa; the volume is drops, so the stain and a blood-culture-bottle or broth inoculation are how you avoid wasting the specimen on five optional agars.

Indigenous Flora Versus Pathogens

These sites are sterile in health, but the needle crosses skin and some collections communicate with mucosa. Skin flora—CoNS, Cutibacterium, Corynebacterium, Bacillus (non-anthracis), and viridans streptococci—can land in the bottle. Mucosal leakage brings Enterobacteriaceae, enterococci, anaerobes, and Candida. Interpretation is not "any growth = pathogen." Use organism identity, quantity, Gram stain agreement, cell count (especially neutrophils), and source.

A single colony of CoNS from a clear pleural fluid with 50 nucleated cells and no organisms on stain is probable contamination. The same CoNS from vitreous after lens surgery, or heavy S. aureus from a synovial fluid with 80,000 neutrophils, is infection. Enterococcus in a clean-catch-looking peritoneal tap from a cirrhotic patient can be true SBP; Enterococcus plus B. fragilis plus two coliforms is not SBP.

Colony Morphology and Identification of Major II.C Pathogens

Keep the same phenotypic toolbox you used in CSF, then add nonfermenters, enterococci, and anaerobes.

Pseudomonas aeruginosa is a straight Gram-negative rod, oxidase-positive, that oxidizes glucose but does not ferment it. Pyocyanin (blue-green) and a grape-like odor are classic on Mueller-Hinton or cetrimide. It is a healthcare and device organism: chest tubes, peritoneal dialysis catheters, and post-traumatic ocular infections. Acinetobacter can appear as plump coccobacilli that mimic Neisseria on a poorly decolorized smear; it is oxidase-negative and grows on MacConkey, which meningococcus does not.

Clostridium perfringens is a large boxcar Gram-positive rod, often without spores in clinical smears, with a double zone of beta-hemolysis on blood agar. Lecithinase (alpha-toxin, phospholipase C) produces an opaque halo on egg-yolk agar (Nagler reaction). Stormy fermentation in litmus milk is historical but still taught. Reverse CAMP with group B Streptococcus is a rapid plate trick. Bacteroides fragilis group organisms are Gram-negative anaerobic rods that grow in 20% bile, form gray-black colonies on Bacteroides bile esculin (BBE) agar, and show the kanamycin-resistant, vancomycin-resistant, colistin-resistant disk pattern. They are bowel organisms; their presence in "ascites" is a red flag for secondary peritonitis.

Anaerobic Workup for Peritoneal Fluid and Abscess

Transport peritoneal fluid and abscess contents in an anaerobic vial or capped syringe (no bubbles). Do not leave a dry swab in room air. Plate Brucella or CDC anaerobic blood agar, kanamycin-vancomycin laked blood agar, and BBE, and incubate in an anaerobic atmosphere promptly. C. perfringens may grow in 24 hours; B. fragilis group often needs 48 hours. If the Gram stain of peritoneal fluid already shows mixed Gram-negative rods plus Gram-positive rods, do not wait for the aerobic MacConkey plate to "look busy" before setting anaerobes—the delay kills the diagnostic pattern of secondary peritonitis.

CAPD (continuous ambulatory peritoneal dialysis) peritonitis is a special peritoneal subset: S. aureus, CoNS, streptococci, P. aeruginosa, and Gram-negative enterics from touch contamination of the catheter. Anaerobes are less common than in bowel-perforation peritonitis, but they are not impossible. Report colony counts or 1+, 2+, 3+ growth as your laboratory defines them; clinicians use that plus effluent cell counts to judge contamination versus infection.

Spontaneous Bacterial Peritonitis Versus Secondary Peritonitis

Spontaneous bacterial peritonitis (SBP) occurs in cirrhotic ascites without a surgically treatable hole in the gut. The culture is typically monomicrobial: E. coli and other Enterobacteriaceae first, then streptococci (including pneumococcus). Anaerobes are uncommon in true SBP. Ascitic fluid PMN counts are often ≥250/µL even when the Gram stain is negative, so a negative stain does not exclude SBP. Secondary peritonitis follows perforation, ischemia, or abscess. Cultures are polymicrobial and include B. fragilis group, C. perfringens, enterococci, and mixed enterics. Isolating B. fragilis from ascites is an exam signal to think secondary, not SBP. That distinction changes surgical management; the laboratory's job is to report the mix and the anaerobes clearly rather than collapsing everything into "Gram-negative rod."

Exam scenario. Paracentesis from a patient with cirrhosis grows E. coli in broth and on MacConkey, one morphology, no anaerobes: SBP pattern. Exam scenario. The next paracentesis grows E. coli, Enterococcus faecium, and a bile-resistant Gram-negative anaerobic rod on BBE: secondary peritonitis until the surgeon proves otherwise.

Pathogenicity and Virulence

Capsule is the invasive-sterile-site theme shared with CSF: pneumococcal polysaccharide, Hib polyribosylribitol phosphate, meningococcal polysaccharide, and E. coli K1. Capsule resists phagocytosis in bloodstream and serous cavities, which is why those organisms seed joints, pleura, and meninges. Toxins matter most for clostridia: C. perfringens alpha-toxin (lecithinase) destroys cell membranes and explains gas gangrene and the double hemolytic zone. Gram-negative LPS endotoxin drives the shock picture of enteric peritonitis. P. aeruginosa adds pili, alginate biofilm, and exotoxins on devices. S. aureus brings protein A, coagulase, and cytotoxins to joints and empyema. You do not need SM-level toxin biochemistry; you do need to pair the virulence factor with the organism the colony tests just identified.

Molecular Methods

Culture-negative septic arthritis, endophthalmitis, and pericarditis are where 16S rRNA gene PCR (sometimes with sequencing) is used on fluid. Syndromic joint-fluid panels exist in some laboratories and may include S. aureus, streptococci, N. gonorrhoeae, and selected Gram-negatives. As with CSF PCR, molecular tests do not replace culture when an isolate is needed for AST, they miss organisms off the panel, and they can detect DNA after treatment. A gonococcal NAAT on synovial fluid can be diagnostic when the organism died in transport—another reason not to refrigerate a suspected Neisseria specimen—but CHOC in CO2 still goes down if volume allows.

Put the whole sterile-fluid algorithm on one card: source and transport, stain, CHOC/BAP/broth ± MacConkey, anaerobes if peritoneal or abscess, interpret flora against cell count, name the II.C pathogens by colony tests, flag SBP versus polymicrobial secondary peritonitis, and use PCR as an adjunct. That is II.C on M.