catalase-positive

**Mechanism/Test:** Catalase is a heme-containing enzyme that detoxifies H2O2, protecting bacteria from reactive oxygen species. In the lab, a colony is mixed with 3% H2O2. Bubbles (O2 gas) mean positive; no bubbles means negative. Avoid picking up blood agar, because RBC catalase gives a false positive.

**Gram-positive cocci:** - Catalase-positive: Staphylococcus (S. aureus is also coagulase-positive; S. epidermidis and S. saprophyticus are coagulase-negative). - Catalase-negative: Streptococcus and Enterococcus.

**Gram-positive rods:** - Catalase-positive: Listeria monocytogenes, Corynebacterium diphtheriae, Bacillus species, Nocardia. - Catalase-negative: Clostridium (also strict anaerobes that lack catalase), Actinomyces, Lactobacillus.

**Gram-negative:** Many are catalase-positive, including Pseudomonas, Neisseria, Haemophilus, Legionella, Escherichia coli, Klebsiella, Serratia, Proteus, Brucella, Campylobacter, Helicobacter, and Bordetella.

**Other:** Fungi such as Aspergillus and Candida are catalase-positive. Mycobacterium tuberculosis is catalase-positive. Isoniazid resistance often involves loss of KatG, a catalase-peroxidase, which activates the drug.

**Clinical significance: chronic granulomatous disease (CGD)** - Cause: NADPH oxidase deficiency (most often X-linked CYBB/gp91phox; also autosomal recessive forms). It impairs the phagocyte respiratory burst, so neutrophils cannot make superoxide and H2O2. - Catalase-negative organisms (e.g., Streptococcus) make their own H2O2. The phagocyte can use that H2O2 for killing, so these organisms are handled normally. - Catalase-positive organisms break down their own H2O2. The phagocyte has no H2O2 source, so these organisms survive and cause recurrent infections. - Classic CGD pathogens ("Staph, Serratia, Nocardia, Burkholderia cepacia, Aspergillus, Klebsiella, E. coli, Pseudomonas"): S. aureus, Serratia, Nocardia, B. cepacia, Aspergillus. - Presentation: recurrent skin, lung, and lymph node abscesses and granulomas, starting in childhood. - Diagnosis: abnormal dihydrorhodamine (DHR) flow cytometry test (decreased fluorescence). The nitroblue tetrazolium (NBT) test is the older test and stays colorless/negative. - Treatment/prophylaxis: TMP-SMX, itraconazole, and interferon-gamma.

**High-yield pearls:** - The catalase test separates Staphylococcus from Streptococcus. - Catalase-positive organisms cause infections in CGD. - Anaerobes often lack catalase, so they are susceptible to oxygen toxicity. - Catalase, superoxide dismutase, and peroxidase are the bacterial defenses against oxidative damage.