oxidase-positive

**Mechanism:** Cytochrome c oxidase (Complex IV) is the terminal enzyme of the bacterial electron transport chain. It transfers electrons from cytochrome c to O2, which is reduced to water. In the lab, the oxidase reagent (tetramethyl-p-phenylenediamine or dimethyl-p-phenylenediamine) acts as an artificial electron donor. Cytochrome c oxidase oxidizes it, and the reagent turns dark blue or purple, usually within 10–30 seconds. The test is done on a filter paper, swab, or strip, using a platinum or plastic loop. Nichrome or iron wire can give false positives.

**Clinically important oxidase-positive organisms (mostly gram-negative):** - *Pseudomonas aeruginosa*: non-lactose fermenter that makes pyocyanin (blue-green pigment) and has a grape-like odor. It is associated with burns, cystic fibrosis, ventilator-associated pneumonia, hot tub folliculitis, malignant otitis externa in diabetics, and ecthyma gangrenosum. - *Neisseria* (*gonorrhoeae*, *meningitidis*): gram-negative diplococci that grow on Thayer-Martin or chocolate agar. Maltose fermentation separates the two: meningococcus ferments maltose and gonococcus does not. - *Vibrio cholerae* (and *V. parahaemolyticus*, *V. vulnificus*): curved rods, grow on TCBS agar, and cause rice-water diarrhea. - *Campylobacter jejuni*: comma or S-shaped rods that grow at 42°C. It is associated with Guillain-Barré syndrome and reactive arthritis. - *Helicobacter pylori*: also urease-positive. It causes peptic ulcers, gastric adenocarcinoma, and MALT lymphoma. - *Legionella pneumophila*: grows on charcoal yeast extract agar with iron and cysteine. - Others: *Pasteurella multocida* (cat or dog bites), *Moraxella catarrhalis*, *Haemophilus* (variable by species), *Aeromonas*, *Plesiomonas*, *Brucella*, *Bordetella pertussis*.

**Oxidase-negative (contrast):** All Enterobacterales are oxidase-negative: *E. coli*, *Klebsiella*, *Enterobacter*, *Serratia*, *Citrobacter*, *Salmonella*, *Shigella*, *Proteus*, and *Yersinia*. Other oxidase-negative organisms are *Acinetobacter baumannii* and *Stenotrophomonas maltophilia*.

**Clinical and board relevance:** - Oxidase status is a quick first test to sort gram-negative rods. Oxidase-negative, lactose-fermenting rods on MacConkey agar suggest Enterobacterales such as *E. coli*. Oxidase-positive, non-lactose-fermenting rods suggest *Pseudomonas*. - Oxidase-positive organisms need to be considered for antipseudomonal coverage. Examples are piperacillin-tazobactam, ceftazidime, cefepime, carbapenems, aminoglycosides, and fluoroquinolones. - Oxidase tests are also used to screen for *Neisseria* in culture. Positive colonies on selective media point to *Neisseria* and prompt further identification. - Oxidase-positive organisms are aerobes or facultative anaerobes that use oxygen as a terminal electron acceptor. Strict anaerobes lack cytochrome oxidase and are oxidase-negative. - Cyanide inhibits cytochrome c oxidase, but this refers to human mitochondrial Complex IV and is a separate concept from the bacterial test.

**Mnemonic:** Oxidase-positive organisms include "PseudoNeisseria Vibrates Campylo-Helically in Legion" (*Pseudomonas, Neisseria, Vibrio, Campylobacter, Helicobacter, Legionella*). A simpler rule is that Enterobacterales are oxidase-negative and most other gram-negative pathogens are oxidase-positive.