Gram stain and shape
Gram-positive rods
Rod-shaped bacteria, also called bacilli, that stain purple: the spore-formers, the diphtheroids, and the branching anaerobes.
How this group is recognized
Gram-positive rods split first on spore formation. The spore-formers divide again by oxygen: Bacillus species grow aerobically, while the Clostridium and Clostridioides species are anaerobes.
The non-spore-formers carry their own study anchors: Listeria with its intracellular behavior, Corynebacterium with its pleomorphic club shapes, and the branching filamentous rods such as Actinomyces that sit near the boundary with the fungi in many course outlines.
PetriKey lists 12 gram-positive rods. Shape is read from each organism's classification; organisms whose card does not state a shape stay on the main Gram-positive page.
All 12
Organisms and their lab clues
Actinomyces israelii Sulfur-granule filamentous anaerobe
Sulfur granules (yellow filament colonies). Branching filaments on Gram stain.
Bacillus anthracis Spore-forming aerobic Gram-positive rod
Nonmotile teaching distinction. Capsule concept.
Bacillus cereus Reheated-rice food poisoning rod
Large box-car Gram-positive rods. Endospore-forming.
Clostridioides difficile Spore-forming anaerobic rod
Toxin testing vocabulary. Spore-forming rod.
Clostridium botulinum Flaccid-paralysis canned-food anaerobe
Spore-forming anaerobe. Toxin detection concept.
Clostridium perfringens Anaerobic spore-forming rod
Double-zone hemolysis vocabulary. Spore-forming rod concept.
Clostridium tetani Terminal-spore tetanus rod
Terminal spore 'tennis-racket' appearance. Obligate anaerobe.
Corynebacterium diphtheriae Club-shaped Gram-positive rod with toxin clue
Palisade or Chinese-letter arrangement. Metachromatic granule vocabulary.
Cutibacterium acnes Acne & prosthesis anaerobe (formerly Propionibacterium)
Slow-growing anaerobe needing prolonged incubation. Gram-positive rod.
Lactobacillus species Doderlein vaginal-flora acid producer
Gram-positive rods (Doderlein bacilli). Catalase-negative.
Listeria monocytogenes Tumbling motility Gram-positive rod
Tumbling motility vocabulary. Narrow beta-hemolysis teaching clue.
Tropheryma whipplei Actinobacterium of PAS-positive Whipple disease
PAS-positive macrophages on duodenal biopsy vocabulary. PCR confirmation on tissue/fluids concept.