Pseudomonas | Bacteriological characters | Culture | Identification

✑ Dr.Midireh.l

☰ Summary :

Ⅰ. Overview | Taxonomy

The genus Pseudomonas is a large and complex heterogeneous group of organisms belonging to the family Pseudomonadaceae. They are subject to continuous taxonomic revision due to improved species identification methodologies (Organisms previously classified in the genus Pseudomonas are now split between the genera Pseudomonas, Burkholderia, Ralstonia, Comamonas, Acidovorax, Delftia, Hyrodenophaga, Brevundimonas, Stenotrophomonas and Xanthomonas).

Twelve species of Pseudomonas were found in clinical specimens: Pseudomonas aeruginosa, Pseudomonas alcaligenes, Pseudomonas fluorescens, Pseudomonas luteola, Pseudomonas mendocina, Pseudomonas monteilii, Pseudomonas mosselii, Pseudomonas oryzihabitans, Pseudomonas pseudoalcaligenes, Pseudomonas putida, Pseudomonas putida.


Pseudomonas sp. are widely distributed in the environment and are often found in moist areas, where they can grow as biofilms attaching to surfaces. They are part of the normal microbiota of the intestinal tract. Mucoid strains cause chronic infections in a high percentage of CF patients (Cystic fibrosis).

Ⅱ. Bacteriological characters

Microscopic examination

Pseudomonas sp. are aerobic, non-spore-forming, motile (one or more polar flagella (swimming motility)), Gram-negative rods, measuring 0.5 to 1 µm by 2 to 7 µm. Bacilli are longer and thinner than Enterobacterales but similar in appearance to other non-fermenters (In ancient cultures may appear slightly pleomorphic, P. putida may appear elongated).

Culture Pseudomonas

The culture of pseudomonas is easy (have very simple nutritional needs), between 10°C and 42°C, on ordinary media (nutrient agar ), selective (Drigalski, Hektoen Enteric Agar, MacConkey Agar or containing cetrimide) or chromogenic media.

Good growth is generally obtained after 24 to 48 hours of incubation. For cultures of patients with cystic fibrosis, it is recommended to maintain the dishes of solid medium at 35 to 37°C for 5 days because certain strains of chronic infections grow very slowly.

Most Pseudomonas aeruginosa organisms are easily recognized on the basis of characteristic flat, spreading colony morphology and have a jagged edge and metallic sheen that is often associated with colony autolysis, but depending on the composition of the media and the type of infection (acute or chronic), the colonies can present different morphologies (mucoids, variants of small "SVG" or "fried egg" colonies).


Different colony morphologies of P. aeruginosa from CF sputum cultured on modified Conradi-Drigalski agar. M, mucoid; NM_1, colony no mucoid 1 (dark green); NM_2, non-mucoid colony 2 (light green); SCV, small colony variant. (Source: 10.1128/9781555817381.ch42.f1)

  • Mucoid isolates of P. aeruginosa from patients with cystic fibrosis may undergo several phenotypic changes, including slow growth, loss of motility, and loss of pigment production.
  • Small colony variants may require prolonged incubation, lack motility, be hyperpiled, adhere to agar surfaces, and exhibit self-aggregating properties in liquid media.

Pseudomonas pigments

The identification of P. aeruginosa strains is facilitated by the very frequent production of an aromatic molecule (o-aminoacetophenone) with a characteristic acacia odor.

Ⅲ. Pseudomonas identification and differentiation between species

Pseudomonas aeruginosa

Pseudomonas fluorescens

❏ Pseudomonas stutzeri

Caractéristiques biochimiques des espèces de Pseudomonas

Biochemical characteristics of Pseudomonas species found in clinical samples (source: manual of clinical microbiology).

Ⅳ. Antibiotic sensitivity and treatment of pseudomonas

What antibiotics is Pseudomonas susceptible to? :

ß-lactams: piperacillin and ticarcillin, with or without inhibitor, ceftazidime, cefepime, aztreonam, imipenem, meropenem, doripenem, Aminosides (except kanamycin), Fluoroquinolones: ciprofloxacin and levofloxacin, colistin and fosfomycin

What antibiotics is Pseudomonas has natural resistance to? :

Penicillins G, A, M, 1st and 2nd generation cephalosporins, oxymino cephalosporins (cefotaxime, ceftriaxone), oral broad-spectrum cephalosporins, Ertapenem, Cyclins including tigecycline, Macrolides, Kanamycin, Old quinolones, Rifampicin, Chloramphenicol, Trimethoprim, Cotrimoxazole , Glycopeptides , Fusidic acid.

Ⅴ. Antibiotics to test "Pseudomonas"

❏ Pseudomonas susceptibility testing is performed on Mueller-Hinton agar, Inoculum: 0.5 McFarland

antibiotics testing of Pseudomonas according to CASFM

standard list : Ticarcillin, Ticarcillin-clavulanic acid, Piperacillin, Piperacillin-tazobactam, Ceftazidime, Cefepime, Ceftolozane-tazobactam, Imipenem, Meropenem, Tobramycin, Amikacin, Ciprofloxacin, Aztreonam, Gentamicin.

Complementary list : Netilmicin, Levofloxacin, Colistin, Fosfomycin, Ceftazidime-avibactam.


  1. Publique healthe england : Health Protection Report February 2020
  2. ECN pilly
  3. Printed Identification Key or Web-Based Identification Guide
  4. Investigation of two-component signaling systems in Pseudomonas aeruginosa and their roles in the mucus barrier - by Benjamin X. Wang
  5. Identification of Pseudomonas species and other Non-Glucose Fermenters - uk gov
  6. Mucoid Pseudomonas in Cystic Fibrosis , Microbiology and Infectious Disease
  7. Revue Francophone des Laboratoires
  8. ASM journales
  9. Standardisation de l'antibiogramme a l'echelle nationale
  10. Lung infection with alginate-producing, mucoid Pseudomonas aeruginosa in cystic fibrosis
  11. Comité de l'Antibiograme de la SFM (CASFM)
  12. EMC : Pseudomonas