Gram stain and shape
Gram-negative rods
The largest branch of the identification tree: enteric rods, nonfermenters, anaerobes, and fastidious species that all stain pink.
How this group is recognized
Gram-negative rods, also called gram-negative bacilli, are the biggest single group in most microbiology courses. The first working split is usually lactose fermentation on MacConkey agar, which separates the classic enteric fermenters from the non-fermenters.
Oxidase status, motility, and growth requirements carry the branch from there: the Enterobacterales cluster on one side, the environmental nonfermenters such as Pseudomonas on another, and the anaerobic and fastidious rods each in their own corner of the workup.
Curved and comma-shaped rods are close cousins but follow a different identification path, so they get their own page.
PetriKey lists 20 gram-negative rods. Shape is read from each organism's classification; organisms whose card does not state a shape stay on the main Gram-negative page.
All 20
Organisms and their lab clues
Aeromonas hydrophila Freshwater rod, wound and gut infection
Oxidase positive (distinguishes from Enterobacterales). Beta-hemolytic on blood agar vocabulary.
Bacteroides fragilis Anaerobic Gram-negative rod with capsule clue
Anaerobic growth concept. Capsule vocabulary.
Bartonella henselae Cat-scratch disease, bacillary angiomatosis
Warthin-Starry silver stain vocabulary. Fastidious, hard-to-culture concept.
Burkholderia cepacia complex Non-fermenter, cystic fibrosis lung pathogen
Oxidase positive (often weak) vocabulary. Non-lactose-fermenting on MacConkey.
Burkholderia pseudomallei Soil non-fermenter causing melioidosis
Bipolar 'safety-pin' staining vocabulary. Oxidase positive concept.
Capnocytophaga species Capnophilic gliding rod of dog and human oral flora
Slow, CO2-dependent (capnophilic) growth vocabulary. Long thin fusiform rods on smear.
Citrobacter species Citrate-using enteric rods, E. coli mimics
Citrate utilization positive vocabulary. Slow lactose fermentation on MacConkey.
Eikenella corrodens HACEK fight-bite Gram-negative rod
Pits / corrodes the agar surface. Capnophilic (needs added CO2).
Enterobacter cloacae Motile enteric rod with inducible AmpC
Lactose fermenter on MacConkey vocabulary. Motile teaching distinction.
Escherichia coli Gram-negative lactose-fermenting rod
MacConkey lactose fermenter in many teaching examples. Oxidase negative.
Fusobacterium necrophorum Lemierre-syndrome anaerobe
Gram-negative anaerobe. Spindle (fusiform) rods on stain.
Klebsiella pneumoniae Encapsulated lactose-fermenting rod
MacConkey lactose-fermenter vocabulary. Capsule concept.
Legionella pneumophila Water-associated intracellular rod
Poor Gram-stain visibility in teaching examples. Urinary antigen vocabulary.
Morganella morganii Urease- and indole-positive nosocomial enteric rod
Urease positive vocabulary. Indole positive (contrast Proteus mirabilis, indole negative).
Proteus mirabilis Urease-positive swarming rod
Urease positive. Swarming motility vocabulary.
Pseudomonas aeruginosa Oxidase-positive nonfermenting rod
Oxidase positive. Non-lactose fermenter.
Salmonella enterica Motile non-lactose fermenting enteric rod
Non-lactose fermenter. H2S vocabulary.
Serratia marcescens Red-pigmented healthcare-associated rod
Red prodigiosin pigment vocabulary. DNase positive concept.
Shigella species Nonmotile non-lactose fermenting rod
Non-lactose fermenter. Nonmotile teaching clue.
Stenotrophomonas maltophilia Multidrug-resistant nosocomial non-fermenter
Oxidase negative vocabulary (contrast with Pseudomonas). DNase positive concept.