Gram stain
Gram-negative bacteria
Bacteria that lose the crystal-violet stain and take the counterstain, reading pink.
Why they stain this way
Gram-negative cell walls have only a thin peptidoglycan layer sitting under an outer membrane. Alcohol dissolves that outer membrane and washes the crystal violet out, so the cells pick up safranin and read pink.
The outer membrane is also where lipopolysaccharide and the porins live, which is why the Gram-negative branch of most identification flows runs through lactose fermentation, oxidase and selective media rather than arrangement alone.
PetriKey lists 42 Gram-negative bacteria. Only bacteria appear here - lab methods and antimicrobials carry gram associations rather than a stain of their own, so they are filed under lab & diagnostics and antimicrobials.
Browse by shape
Shape is the next branch after the stain. Each Gram-negative shape group has its own study page with the identifying lab clues for every member.
All 42
Gram-negative organisms
Acinetobacter baumannii
Healthcare-associated nonfermenting coccobacillus
Gram −Aeromonas hydrophila
Freshwater rod, wound and gut infection
Gram −Anaplasma phagocytophilum
Intracellular tick-borne bacterium of granulocytes
Gram −Bacteroides fragilis
Anaerobic Gram-negative rod with capsule clue
Gram −Bartonella henselae
Cat-scratch disease, bacillary angiomatosis
Gram −Bordetella pertussis
Small respiratory coccobacillus with toxin vocabulary
Gram −Brucella species
Undulant fever, unpasteurized dairy zoonosis
Gram −Burkholderia cepacia complex
Non-fermenter, cystic fibrosis lung pathogen
Gram −Burkholderia pseudomallei
Soil non-fermenter causing melioidosis
Gram −Campylobacter jejuni
Curved motile Gram-negative rod
Gram −Capnocytophaga species
Capnophilic gliding rod of dog and human oral flora
Gram −Citrobacter species
Citrate-using enteric rods, E. coli mimics
Gram −Coxiella burnetii
Q fever, aerosol zoonosis, no vector needed
Gram −Ehrlichia chaffeensis
Obligate intracellular tick-borne bacterium of monocytes
Gram −Eikenella corrodens
HACEK fight-bite Gram-negative rod
Gram −Enterobacter cloacae
Motile enteric rod with inducible AmpC
Gram −Enterotoxigenic E. coli (ETEC)
Toxin-driven watery traveler's diarrhea pathotype
Gram −Escherichia coli
Gram-negative lactose-fermenting rod
Gram −Francisella tularensis
Tularemia, rabbit/tick zoonosis, cysteine need
Gram −Fusobacterium necrophorum
Lemierre-syndrome anaerobe
Gram −Haemophilus influenzae
Small Gram-negative coccobacillus with X/V factors
Gram −Helicobacter pylori
Curved urease-positive gastric rod
Gram −Kingella kingae
Fastidious HACEK coccobacillus of young children
Gram −Klebsiella pneumoniae
Encapsulated lactose-fermenting rod
Gram −Legionella pneumophila
Water-associated intracellular rod
Gram −Moraxella catarrhalis
Oxidase-positive Gram-negative diplococcus
Gram −Morganella morganii
Urease- and indole-positive nosocomial enteric rod
Gram −Neisseria gonorrhoeae
Oxidase-positive Gram-negative diplococcus
Gram −Neisseria meningitidis
Gram-negative diplococci
Gram −Pasteurella multocida
Cat/dog bite coccobacillus, rapid cellulitis
Gram −Proteus mirabilis
Urease-positive swarming rod
Gram −Pseudomonas aeruginosa
Oxidase-positive nonfermenting rod
Gram −Salmonella enterica
Motile non-lactose fermenting enteric rod
Gram −Serratia marcescens
Red-pigmented healthcare-associated rod
Gram −Shiga toxin-producing E. coli (STEC/EHEC)
Shiga-toxin pathotype linked to HUS
Gram −Shigella species
Nonmotile non-lactose fermenting rod
Gram −Stenotrophomonas maltophilia
Multidrug-resistant nosocomial non-fermenter
Gram −Vibrio cholerae
Comma-shaped oxidase-positive rod
Gram −Vibrio parahaemolyticus
Salt-loving seafood gastroenteritis vibrio
Gram −Vibrio vulnificus
Marine vibrio, wound and shellfish sepsis
Gram −Yersinia enterocolitica
Cold-tolerant enteric rod, pseudoappendicitis
Gram −Yersinia pestis
Flea-borne plague, safety-pin bipolar rod
Gram −