The Gram stain in logarithmic phase (exponential phase), acinetobacter appear as small Gram-negative coccobacilli, measuring 1.0-1.5 µm by 1.5-2.5 µm, singly or in pairs. They are very similar in morphology to Neisseria spp. (Easily differentiated by oxidase text), but generally become more coccoid in stationary phase. Because they are difficult to discolor, they may be misidentified as Gram-variable or Gram-positive “staph-like” cocci.
Although rare, patients with serious infections may not receive appropriate treatment in such cases if antibiotics are reduced based on Gram stain alone
Acinetobacter appearing as Gram-positive cocci.
Acinetobacter do not ferment glucose, and are strictly aerobic, so they will not change the color of the TSI medium, unlike, for example, Staphylococcus which performs acid fermentation..
To better appreciate the bacterial coloration, it is advisable to put colonies in a nutrient broth and incubate it in a water bath for 30min to 1 hour..
The 'KOH' potassium hydroxide test can be used as a confirmatory test for Gram staining The formation of a chain (DNA) in 3% of KOH indicates that the isolate is a Gram-negative organism (Gram-negative cell walls are broken down by 3% KOH and in turn release viscoid chromosomal material which causes the suspension to become thick and stringy.)
KOH string test positive
- European Society of Clinical Microbiology and Infectious Diseases : Acinetobacter ursingii masquerading as Gram-positive cocci
- The Open Microbiology Journal ; Misinterpretation of Gram Stain from the Stationary Growth Phase
- ASM journals