A beta-haemolytic urinary isolate of Escherichia coli, demonstrating the considerable phenotypic diversity of this major uropathogen and the striking colony appearance that may accompany production of a haemolytic virulence factor.
Basic Characteristics
Taxonomy
Domain: Bacteria
Phylum: Pseudomonadota (Proteobacteria)
Class: Gammaproteobacteria
Order: Enterobacterales
Family: Enterobacteriaceae
Genus:Escherichia
Species:Escherichia coli
Microscopy & Gram Stain
Gram-negative rods
Cells occur mainly singly, with pairs and short arrangements also possible
Abundant leukocytes may be visible in urine during active infection
Oxygen Relationship
Facultatively anaerobic
Rapid Identification Tests
Oxidase: negative
Catalase: positive
Indole: usually positive
Lactose fermentation: usually positive
Motility: usually positive
Haemolysis on blood agar: marked beta-haemolysis in this isolate
Ecology and Clinical Relevance
Natural Habitat
Human and animal intestinal tract
Common member of the normal colonic microbiota
May colonize the perineal and periurethral region before ascending into the urinary tract
Widely present in environments contaminated by faecal material
Common Clinical Specimens
Urine
Blood cultures
Wound and tissue specimens
Intra-abdominal material
Stool when intestinal pathogenic strains are suspected
Cerebrospinal fluid in neonatal infection
Clinical Significance
The most frequent bacterial cause of community-acquired urinary tract infection
Important cause of catheter-associated and healthcare-associated urinary infection
May cause cystitis, pyelonephritis, urosepsis, and bloodstream infection
Beta-haemolytic urinary isolates may produce alpha-haemolysin, a virulence factor present in a subset of uropathogenic strains
Differential Considerations
Klebsiella pneumoniae, Citrobacter, Enterobacter, and other lactose-fermenting Enterobacterales
Proteus mirabilis and other common urinary Gram-negative rods
Haemolytic Gram-negative isolates such as some Aeromonas species, which are typically oxidase positive
Colony appearance and beta-haemolysis are not species-specific and must be combined with biochemical or instrumental identification
Ten microlitres of midstream urine were inoculated and streaked onto blood agar. The specimen was obtained from a 40-year-old woman with the diagnosis N30.2, other chronic cystitis. A pure culture of Escherichia coli was recovered. Compared with the preceding non-haemolytic urinary isolate, this strain shows striking beta-haemolysis and was present at a relatively lower concentration in the original specimen. Culture conditions: 24 hours at 36 °C in ambient air.
Beta-haemolytic colonies of Escherichia coli on blood agar after 24 hours of incubation at 36 °C in ambient air. Several drops of 3% hydrogen peroxide were applied to the culture, producing rapid bubbling caused by the release of oxygen during a positive catalase reaction. E. coli is characteristically catalase-positive. Catalase activity is common among members of the order Enterobacterales, although the test is now used mainly for demonstration rather than for routine identification of these bacteria.
Close-up view of beta-haemolytic Escherichia coli colonies on blood agar after 24 hours of incubation at 36 °C in ambient air. The colonies are smooth, convex, grey-white, and round with entire margins. Each colony is surrounded by a prominent zone of complete haemolysis. This phenotype contrasts sharply with the non-haemolytic appearance of many urinary E. coli isolates and demonstrates the considerable morphological variability found within the species.
Gram-stained smear of the original urine specimen, from which a pure culture of Escherichia coli was subsequently isolated. Numerous Gram-negative rods are visible together with abundant leukocytes. The marked inflammatory cellular response supports an active urinary tract infection and contrasts with specimens in which significant bacteriuria may be accompanied by only limited leukocyturia.
Diagnostic and Clinical Notes
Escherichia coli is a highly diverse species. Most strains colonize the intestinal tract without causing disease, whereas specialized intestinal and extraintestinal lineages possess combinations of virulence factors that permit infection at other anatomical sites.
Uropathogenic E. coli is the leading bacterial cause of urinary tract infection. Infection usually develops by an ascending route after organisms from the intestinal microbiota colonize the periurethral region and enter the urinary tract.
The prominent beta-haemolysis seen in this isolate may reflect production of alpha-haemolysin (HlyA), an RTX-family pore-forming toxin found in a subset of uropathogenic E. coli. HlyA can injure epithelial and immune cells and may contribute to inflammation and tissue damage during urinary infection.
Visible haemolysis is a useful descriptive feature but is not sufficient to define a strain as uropathogenic or to determine its complete virulence profile. Haemolytic activity varies with the strain, culture conditions, toxin expression, and the presence of other haemolytic determinants.
In the original urine specimen, numerous Gram-negative rods were accompanied by abundant leukocytes. This combination supports an active inflammatory process, but the clinical significance still depends on quantitative culture, symptoms, specimen quality, and the patient’s overall condition.
Laboratory Identification
Colony Morphology
On blood agar after 18–24 hours at 35–37 °C, this isolate forms smooth, convex, grey-white colonies with entire margins and a broad zone of complete beta-haemolysis. Colony morphology among Escherichia coli strains is highly variable, and many urinary isolates are non-haemolytic. On lactose-containing differential media, most isolates ferment lactose and produce coloured colonies.
Microscopy
Gram staining shows Gram-negative rods, usually occurring singly. The original urine smear in this case contains numerous bacteria together with abundant leukocytes, supporting a substantial inflammatory response in the urinary tract.
Key Identification Clues
Gram-negative rods
Facultatively anaerobic growth
Oxidase negative
Catalase positive
Usually lactose fermenting
Usually indole positive and motile
Marked beta-haemolysis in this isolate, but haemolysis is variable within the species
Modern Identification Methods
Routine species identification is commonly achieved using MALDI-TOF mass spectrometry or automated biochemical systems. Molecular testing may be used when characterization of virulence genes such as hlyA, intestinal pathotypes, Shiga toxin production, resistance mechanisms, or epidemiologically important clones is required.
Antibiotic Characteristics
Antimicrobial susceptibility in Escherichia coli varies substantially between patients, geographical regions, clinical settings, and individual lineages.
Urinary isolates may be resistant to commonly used agents, including aminopenicillins, trimethoprim-sulfamethoxazole, and fluoroquinolones. Local resistance data are therefore important when empirical treatment is considered.
Extended-spectrum beta-lactamase production is an important resistance mechanism in both community and healthcare-associated isolates. Some strains also acquire carbapenemases or other mechanisms that further restrict treatment options.
Note: The beta-haemolytic appearance of this isolate does not predict its antimicrobial susceptibility. Treatment decisions should be guided by susceptibility testing, the site and severity of infection, and the clinical context.
