Microorganisms can produce different types of volatile compounds which may give characteristics smell, pleasant scent or pungent odor. Production of these volatile chemicals depends on the metabolic characteristics of that particular organism. Sometimes scent or odor produced by bacteria can give important clue in the identification of microorganisms, but it is not a reliable rule.
You may have seen Microbiologist/technician trying to “smell the difference” between bacterial culture. Though, some laboratory personnel regard sniffing as a useful tool other regards it as a biohazard (1). I suggest you to be cautious while sniffing culture plates. Sniffing culture plates is not a good idea and you may contract a disease (2).
Table: Characteristics odor of selected microbes
|Alkaligenes faecalis||Freshly cut apples|
|Citrobacter spp.||Dirty sneakers|
|Clostridium difficile||Putrid, fecal|
|Haemophilus spp.||Wet fur|
|Nocardia spp.||Musty basement|
|Pasterurella multocida||Pungent (indole)|
|Pigmented Bacterioides group||Acrid|
|Proteus spp.||Burnt chocolate|
|Pseudomonas aeruginosa||Grape juice|
|Staphylococcus spp.||Dirty sneakers|
|Certain viridans group Streptococcus spp.||Butter/butterscotch|
|Streptomyces spp.||Musty basement|
- Pungent/Unpleasant smell
Anaerobes are particularly pungent due to their reliance on sulfhydryl compounds to maintain redox balance. When an anaerobic infection is suspected, the specimen is often foul-smelling. Gram-negative anaerobes are often responsible for ‘morning breath’.
- Bacterioides fragilis: Pus containing Bacteroides species has a very unpleasant smell.
- Under anaerobic conditions, Clostridium perfringens multiplies and produces alpha toxin and other toxins which result in therapid destruction of tissue carbohydrate with the production of gas in decaying tissues, particularly muscle. The affected tissue is foul-smelling.
- Yeast like smell: The colonies of Candida albicans have a distinctive yeast smell.
- Smell like a soil
Certain cyanobacteria and actinomycetes synthesize geosmin, a volatile chemical that is reminiscent of potting soil.
- Rotten cooked fishy odor: Proteus mirabilis produces a very distinct fishy odour. On Salmonella-Shigella (SS) agar, Proteus usually smells like “rotten cooked fish”.
- Sweet grape-like scent
Pseudomonas aeruginosa produces a sweet grape-like scent, so wound dressings and agar plates are often sniffed for organism identification. Pseudomonas aeruginosa can famously generate a “grape juice” smell in infected burn patients (3). Cultures of Pseudomonas have a distinctive smell due to the production of 2-aminoacetophenone.
- Ammoniacal smell
- Woman infected with Gardnerella complaints of grey, offensive, fishy ammoniacal smell. Fishy ammoniacal smell becomes more intense after adding a few drops of 10% potassium hydroxide.
- pseudomallei cultures give off an ammoniacal smell.
- Bleach-like odor: Eikenella corrodens, a gram-negative rod, responsible for wound infections gives a bleach-like odor when grown in Blood Agar or Chocolate Agar.
- Caramel odor: Streptococcus milleri produces diacetyl (caramel odor). The detection of diacetyl (caramel odor) can be used in presumptive identification of the “Streptococcus milleri” group (4).
Further Reading and references
- Sniffing Bacterial Cultures on Agar Plates: a Useful Tool or a Safety Hazard?
- Brucellosis from sniffing bacteriological cultures
- A Microbe By Any Other Name Would Smell As Sweet
- Detection of diacetyl (caramel odor) in presumptive identification of the “Streptococcus milleri” group