Bacteria exposed to subinhibitory and sublethal concentrations of antibiotics may show a number of morphologic changes on Gram stain.
Change
Seen in
mottled Gram positivity
Gram-positive bacteria (due to antibiotics affecting the cell wall)
loss of acid-fastness
mycobacteria
filamentous forms
Gram-negative bacteria (due to inhibition of enzymes that initiate septation)
rod-shaped forms
Streptococcus pneumoniae
granular or bipolar staining
Enterobacteriaceae (may be associated with density and distribution of ribosomes)
focal dilatations (globules)
Gram negative bacteria; Streptococcus pneumoniae
tetrads or conglomerates
diplococci (such as Neisseria gonorrhoeae)
enlargement (giant forms)
Staphylococcus aureus (inhibition of cytoplasmic division)
greater retention of crystal violet-iodine complex
Staphylococcus aureus (due to thickening of cross walls)
Implications:
(1) These findings indicate subinhibitory concentration of antibiotics.
(2) The type of organism causing an infection may be misidentified (for example, Streptococcus pneumoniae).
(3) Therapy may be assumed to be more effective than it really is (for example, acid fast bacteria may disappear if acid-fastness is lost).
(4) Cultures may give false negative results if media is hypotonic. In addition, cultures should be held for a longer period (in the event of delayed proliferation).
To read more or access our algorithms and calculators, please log in or register.
