Staphylococcus pasteuri is a yellow-pigmented, coagulase-negative staphylococcal species that may occur in human clinical material and can resemble more familiar staphylococci when colony colour or haemolysis dominates the first visual impression on blood agar.
Basic Characteristics
Taxonomy
Domain: Bacteria
Phylum: Bacillota (Firmicutes)
Class: Bacilli
Order: Bacillales
Family: Staphylococcaceae
Genus:Staphylococcus
Species:Staphylococcus pasteuri
Microscopy & Gram Stain
Gram-positive cocci
Cells occur singly, in pairs, and in irregular clusters
Microscopic morphology is typical of staphylococci but not species-specific
Oxygen Relationship
Facultatively anaerobic
Grows readily under routine aerobic blood agar conditions
Not regarded as a fastidious species in routine laboratory work
Rapid Identification Tests
Catalase: positive
Coagulase: negative
Novobiocin: usually susceptible
Colony pigmentation: often yellow, but variable in intensity
Haemolysis on blood agar: usually absent, but some strains may be beta-haemolytic
Ecology and Clinical Relevance
Natural Habitat
Originally described from human, animal, and food-associated specimens
May occur on human skin or in skin-associated clinical material
Can also be encountered in environmental or mixed microbiological settings
Common Clinical Specimens
Ear canal swabs and other skin-associated material
Occasional wound or urine specimens
Blood culture material in uncommon cases
Mixed cultures in which colony colour helps draw attention to the isolate
Clinical Significance
Uncommon opportunistic pathogen in humans
Usually encountered as a coagulase-negative staphylococcal isolate of uncertain or context-dependent significance
May be clinically relevant in selected infections, including invasive disease reported in the literature
Can be overlooked or misjudged when mixed with other skin-associated staphylococci
Differential Considerations
Staphylococcus saprophyticus, because both may appear yellowish but differ in novobiocin susceptibility
Staphylococcus aureus, when pigmentation is strong or when haemolysis is present
Staphylococcus epidermidis and Staphylococcus auricularis in mixed plate cultures
Species-level identification should not rely on colony colour alone
Staphylococcus pasteuri isolated from an ear canal swab from a 66-year-old woman with otitis externa, unspecified. A strongly yellow-pigmented colony from the primary culture was subcultured onto blood agar and incubated for 24 hours at 36 °C in ambient air. The plate shows a mixture of intensely yellow colonies and colonies with much weaker pigmentation. Unlike Staphylococcus saprophyticus, S. pasteuri is characteristically susceptible to novobiocin.
Mixed culture of three coagulase-negative staphylococcal species on blood agar after 24 hours of incubation at 36 °C. Yellow-pigmented colonies of Staphylococcus pasteuri are visible together with greyish colonies of Staphylococcus epidermidis and white colonies of Staphylococcus auricularis. Such mixed growth illustrates how colony colour can provide a useful first clue during plate reading, even though final species identification requires additional laboratory methods.
Close-up view of Staphylococcus pasteuri colonies on blood agar after 24 hours at 36 °C in ambient air. In this case, the yellow pigmentation is striking, although some colonies show the colour less intensely than others. The isolate is non-haemolytic, but some strains may produce beta-haemolysis and thereby resemble Staphylococcus aureus. This combination of variable pigmentation and haemolysis can complicate presumptive visual identification on primary culture plates.
Gram-stained smear prepared from a colony of Staphylococcus pasteuri grown on blood agar. The organism appears as Gram-positive cocci occurring singly, in pairs, and in clusters. This microscopic pattern is typical of staphylococci and is useful for confirming the general group of the isolate, although species-level identification must rely on additional phenotypic or instrumental methods.
Diagnostic and Clinical Notes
Staphylococcus pasteuri is a useful teaching species because the colony phenotype can be much more visually striking than one might expect for a coagulase-negative staphylococcus. In this page set, the isolate produces strong yellow pigmentation, while the intensity of colour varies from colony to colony on the same plate.
The mixed culture shown here is particularly instructive. Yellow colonies of S. pasteuri are present alongside greyish colonies of Staphylococcus epidermidis and white colonies of Staphylococcus auricularis, illustrating how colony colour can guide first impressions during plate reading without providing definitive identification.
The practical differential diagnosis includes Staphylococcus saprophyticus and Staphylococcus aureus. The former may also be yellowish, whereas the latter becomes a concern when pigment is combined with haemolysis. In the present isolate, absence of haemolysis is a helpful clue, but this feature is not absolute for the species.
Novobiocin susceptibility is a useful practical discriminator in this context. Unlike S. saprophyticus, S. pasteuri is characteristically susceptible, which can help refine the diagnostic impression when yellow-pigmented coagulase-negative staphylococci are encountered.
The ear canal origin of the isolate also matters clinically. In otitis externa and related sampling, coagulase-negative staphylococci may represent colonizing or accompanying flora, so species identification should always be interpreted together with specimen type, quantity of growth, and the broader microbiological context.
Laboratory Identification
Colony Morphology
On blood agar, Staphylococcus pasteuri forms smooth, convex, round colonies with entire margins. Many isolates show yellow pigmentation, but the intensity may vary considerably even within a single culture. The isolate shown here is non-haemolytic, although some strains may produce beta-haemolysis and thereby resemble more familiar staphylococcal species.
Microscopy
Gram staining shows Gram-positive cocci occurring singly, in pairs, and in clusters. This appearance confirms the general staphylococcal morphology but does not allow reliable distinction of S. pasteuri from other coagulase-negative staphylococci or from more familiar species with overlapping plate phenotypes.
Key Identification Clues
Gram-positive cocci in clusters
Catalase positive
Coagulase negative
Often yellow-pigmented colonies
Usually non-haemolytic, but beta-haemolysis may occur in some strains
Typically susceptible to novobiocin
Species-level identification should be confirmed instrumentally when available
Modern Identification Methods
Species-level identification is best supported by MALDI-TOF mass spectrometry with an up-to-date reference database, or by molecular methods when necessary. This is particularly relevant for Staphylococcus pasteuri because the organism can overlap phenotypically with better known pigmented coagulase-negative staphylococci and, in some cases, even with Staphylococcus aureus.
Antibiotic Characteristics
Staphylococcus pasteuri should not be assigned a susceptibility pattern on the basis of species name or colony appearance alone. As with other coagulase-negative staphylococci, antimicrobial susceptibility may vary between isolates.
Novobiocin susceptibility is diagnostically useful but should not be mistaken for a therapeutic guide. Clinical treatment decisions should always rely on formal antimicrobial susceptibility testing of the individual isolate.
When the organism is recovered from mixed clinical material, the therapeutic interpretation depends not only on susceptibility results but also on deciding whether S. pasteuri is acting as a pathogen, a copathogen, or a less significant accompanying organism.
Note: Antimicrobial therapy should always be guided by laboratory susceptibility testing, specimen context, and overall clinical evaluation.
