Aerococcus

Aerococcus is a genus of Gram-positive, catalase-negative, microaerophilic cocci that typically occur in pairs, tetrads, or irregular clusters, measuring 1–2 µm in diameter, and are commonly isolated from the air and dust in occupied environments such as schoolrooms and streets.^[1] These bacteria are aerobic or facultatively anaerobic, non-motile, oxidase-negative, and capable of growth at temperatures between 22°C and 37°C, with tolerance to high bile concentrations (up to 40%) and alkaline pH (up to 9.6), but they do not grow at 45°C or form chains like streptococci.^[1] They produce α-hemolysis on blood agar, resulting in a greening effect, and require complex nutritional needs due to their fastidious nature, often leading to misidentification as viridans group streptococci or enterococci in clinical settings.^[2] The genus Aerococcus was first proposed in 1953 by Williams, Hirsch, and Cowan to classify a group of morphologically distinct, air-prevalent cocci that differed from established genera like Streptococcus and Staphylococcus.^[1] Accurate identification has been challenging historically due to unreliable biochemical tests, but modern methods such as matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) and 16S rRNA gene sequencing now enable rapid and precise differentiation.^[3] As of 2025, the genus comprises 13 recognized species.^[4] While Aerococcus species are generally of minor interest in food technology as lactic acid bacteria, they are increasingly acknowledged as emerging human pathogens, particularly in immunocompromised or elderly individuals.^[5] The most clinically significant species are A. urinae and A. sanguinicola, which are frequently isolated from urine and blood, accounting for approximately 55–66% and 26–46% of human Aerococcus isolates, respectively.^[5] These bacteria cause urinary tract infections (UTIs) in 0.2–0.8% of positive cultures, often in older men with underlying urological conditions, and can lead to invasive infections such as bacteremia, infective endocarditis, urosepsis, peritonitis, and osteomyelitis.^[3] A. viridans, first reported as a human pathogen in 1967, is less common but associated with similar infections, including endocarditis and arthritis, predominantly in patients over 65 years old.^[2] Epidemiologically, Aerococcus-related infections occur at an estimated rate of 3–20 cases per million population annually, with improved detection via MALDI-TOF MS contributing to higher recognition rates since the 2010s.^[5] Prognosis for bacteremia shows 17% 30-day all-cause mortality, with infective endocarditis carrying higher fatality risks (14–50%).^[6] Treatment typically involves penicillin or ceftriaxone for invasive cases, often combined with aminoglycosides for synergy in endocarditis, while susceptibility to trimethoprim-sulfamethoxazole and fluoroquinolones varies for UTIs.^[3]

Taxonomy and Etymology

Etymology

The genus name Aerococcus was proposed in 1953 by R. E. O. Williams, Ann Hirsch, and S. T. Cowan to describe a group of aerobic Gram-positive cocci isolated from the air in occupied environments and from dust samples.^[1] The name derives from the Ancient Greek aēr (ἀήρ), meaning "air" or "gas," combined with kokkos (κόκκος), meaning "grain" or "berry," and the New Latin suffix -coccus, denoting berry-shaped bacteria; this etymology highlights the genus's initial discovery in airborne and particulate settings.^[4]

Classification and History

The genus Aerococcus belongs to the phylum Bacillota, class Bacilli, order Lactobacillales, and family Aerococcaceae.^[4][7] This placement reflects its phylogenetic position among Gram-positive, catalase-negative cocci related to lactic acid bacteria.^[8] Aerococcus was first described in 1953 by Williams, Hirsch, and Cowan, who proposed the genus for aerobic, Gram-positive cocci isolated from air in occupied rooms and from dust samples.^[9] The type species, A. viridans, was established at that time based on these environmental isolates.^[9] Subsequent emendations to the description of A. viridans have refined its characteristics, incorporating data from additional strains and molecular analyses.^[10] Taxonomic revisions in the genus have included reclassifications from related genera, notably the transfer of Pediococcus urinaeequi (originally described in 1934) to Aerococcus urinaeequi comb. nov. in 2005, based on 16S rRNA gene sequencing and phenotypic similarities such as fatty acid profiles and growth patterns.^[11][8] Other species have been incorporated through similar phylogenetic and biochemical evaluations, expanding the genus beyond its original environmental focus; as of 2025, there are 13 validly published species in the genus.^[8][4]

Description

Morphology and Colony Characteristics

Aerococcus species are Gram-positive cocci with a spherical to ovoid shape, measuring approximately 1–2 μm in diameter.^[1] These bacteria divide in two planes at right angles, resulting in characteristic tetrads and irregular clusters, distinguishing them from chain-forming streptococci.^[12] Unlike staphylococci, they do not form typical grape-like clusters and are catalase-negative, which aids in their differentiation from similar genera.^[12] Aerococcus cells are non-motile and non-spore-forming, exhibiting no flagella or endospores under standard microscopic examination.^[13] In Gram stains from clinical or culture samples, they appear as pairs, tetrads, or loose clusters of Gram-positive cocci. On blood agar, Aerococcus colonies are small, typically 0.5–2 mm in diameter after 24–48 hours of incubation, and exhibit a convex, shiny, and transparent appearance.^[1] They display α-hemolysis, producing a greenish discoloration around the colonies due to partial hemolysis of red blood cells, resembling viridans group streptococci.^[14] At 24 hours, colonies may appear more streptococcus-like, transitioning to an enterococcus-like morphology by 48 hours.^[15]

Physiological Properties

Aerococcus species exhibit microaerophilic growth preferences but are facultatively anaerobic, enabling adaptation to varying oxygen levels in their environments. Growth occurs at temperatures between 22°C and 37°C, with no growth at 45°C, aligning with human body conditions, and they tolerate alkaline pH up to 9.6, supporting their potential as opportunistic pathogens.^[1] These bacteria are non-motile, non-spore-forming Gram-positive cocci that demonstrate tetrad formation particularly in broth cultures.^[16] Biochemically, Aerococcus species are catalase-negative, with occasional weak reactions in some strains due to pseudocatalase activity, and consistently oxidase-negative, lacking cytochrome enzymes. Most species, such as A. viridans and A. sanguinicola, test positive for pyrrolidonyl arylamidase (PYR) activity, aiding in differentiation from related genera, though A. urinae is PYR-negative.^[17][15] Aerococcus displays a fermentative metabolism, primarily producing lactic acid from glucose fermentation without gas production, consistent with its classification among lactic acid bacteria-like organisms. Arginine dihydrolase activity is variable across species, serving as a key differentiator in identification schemes, with positive reactions in some like A. viridans.^[18][19][12] These bacteria tolerate moderate salt concentrations, growing in up to 6.5% NaCl for most species and as high as 10% for A. viridans, and can withstand 40% bile, as demonstrated in bile esculin tests where some hydrolyze the substrate. Unlike certain enterococci that may exhibit vancomycin resistance, Aerococcus species remain susceptible to vancomycin, with minimum inhibitory concentrations typically ≤1 μg/mL.^[16][20]

Habitat and Distribution

Environmental Sources

Aerococcus species were initially isolated in the 1950s from air samples in occupied rooms, dust, and vegetation, marking the genus's recognition as an environmental bacterium. The type species, Aerococcus viridans, was described from these airborne and particulate sources, highlighting its association with suspended particles in indoor and natural settings.^[9] Subsequent isolations have confirmed the presence of Aerococcus in soil, where strains such as A. agrisoli have been recovered from paddy fields, and in various terrestrial environments producing hydrogen peroxide.^[21][22] The genus is also found in plant-associated habitats, including vegetation, and marine environments, with A. viridans linked to crustaceans and seawater.^[17] In animal ecosystems, Aerococcus occurs as part of indigenous microbiota, with rare veterinary isolations such as A. vaginalis from bovine vaginal mucosa and A. viridans implicated in lobster gaffkemia and bovine mastitis.^[23][24][25] Environmental water sources have yielded occasional isolates, underscoring the bacterium's wide but sporadic distribution.^[17] Overall, Aerococcus maintains low abundance in natural ecosystems, often functioning as transient flora rather than dominant members of microbial communities.^[26] This environmental ubiquity has contributed to its later recognition as an opportunistic pathogen in humans.^[27]

Human Association

Aerococcus species, particularly A. urinae and A. viridans, are infrequently detected as part of the indigenous human microbiota, primarily in the urinary tract where they occur at low abundance as non-dominant members of the urobiome.^[28] They have also been noted in the oral flora and, to a lesser extent, associated with skin and respiratory sites through environmental exposure rather than established colonization.^[29] In the gastrointestinal tract, their presence remains minimally documented and typically transient.^[17] Isolation of Aerococcus from urine samples is relatively common in asymptomatic individuals, especially among elderly males with predisposing urinary conditions such as prostatic hypertrophy or indwelling catheters, where it may represent colonization rather than active infection.^[30] Studies indicate that such bacteriuria occurs more frequently in this demographic, often without clinical symptoms, highlighting its role in the normal or perturbed urinary microbiome.^[29] Tetrad-forming Gram-positive cocci consistent with Aerococcus morphology can occasionally appear in Gram stains from non-invasive urine samples in these cases.^[17] While not a primary colonizer, Aerococcus functions as an opportunistic pathogen, particularly in immunocompromised hosts or individuals with underlying urological issues, where low-level carriage can precede dissemination under favorable conditions.^[31] Its presence in such populations underscores a shift from commensal to pathogenic potential without being a dominant resident in healthy microbiota.^[28] Aerococcus has been detected in hospital environments through air sampling, suggesting a nosocomial transmission risk via airborne dissemination in clinical settings.^[32] Isolates from hospital air, including A. viridans, indicate environmental persistence that could facilitate opportunistic colonization in vulnerable patients.^[33]

Pathogenicity and Clinical Relevance

Associated Infections

Aerococcus species are emerging pathogens primarily associated with urinary tract infections (UTIs), bacteremia, and infective endocarditis, particularly in elderly patients with underlying urological conditions.^[34] Aerococcus urinae is the most commonly implicated species in these infections, often originating from the urinary tract and leading to invasive disease such as urosepsis or endocarditis in older males with prostate hypertrophy or other urinary abnormalities.^{[34] [35]} The epidemiology of Aerococcus infections has gained recognition since the 1990s, coinciding with improved laboratory identification techniques that distinguish aerococci from streptococci or staphylococci.^[34] Incidence of aerococcal bacteremia is estimated at 1.48 per 100,000 person-years, with higher rates reported in northern Europe.^{[36] [37]} Risk factors include advanced age (median around 74–82 years, predominantly >65 years), male sex (70–80% of cases), and comorbidities such as diabetes, chronic kidney disease, neurologic disorders, and prior hospitalizations.^{[36] [37] [35]} Secondary infections caused by Aerococcus are less common but include skin and soft tissue infections, such as perigenital necrotizing fasciitis, spontaneous bacterial peritonitis, and rare cases of pneumonia, meningitis, or osteomyelitis.^{[38] [39] [40]} These typically occur in immunocompromised individuals or as complications of primary bacteremia. Initial misidentification as streptococci or staphylococci in cultures can delay recognition of these secondary manifestations.^[34] Mortality rates for aerococcal bacteremia range from 10–24%, with higher rates (up to 50%) observed in cases of infective endocarditis, often due to delayed diagnosis and patient comorbidities.^{[41] [42]} Despite this, prognosis is generally favorable with prompt treatment, particularly in non-endocarditis bacteremia.^[34]

Diagnosis and Treatment

Diagnosis of Aerococcus infections typically begins with Gram staining, which reveals Gram-positive cocci arranged in tetrads or clusters, often appearing in pairs or irregular clusters under microscopic examination.^[43][5] These organisms are catalase-negative and can be cultured on blood agar, where they form small colonies exhibiting α-hemolysis after 24-48 hours of incubation in 5% CO₂.^[41][43] Biochemical testing further aids identification, with Aerococcus species negative for catalase and variable for pyrrolidonyl aminopeptidase (PYR) (negative for A. urinae, positive for A. viridans); additional tests such as β-glucuronidase activity (positive for A. urinae) and leucine aminopeptidase production help differentiate species like A. urinae and A. sanguinicola.^[41][43][15] For definitive confirmation, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) or 16S rRNA gene sequencing is recommended, as these molecular methods provide rapid and accurate species-level identification.^[41][5] A major challenge in diagnosing Aerococcus infections is their frequent misidentification by automated commercial systems, such as VITEK 2 or API strips, which often classify them as viridans group streptococci, enterococci, or even staphylococci due to morphological and biochemical similarities.^[43][5] This leads to underrecognition or dismissal as contaminants, particularly in blood cultures, necessitating manual review of Gram stains and biochemical profiles or escalation to molecular techniques for resolution.^[41][43] Aerococcus species demonstrate high susceptibility to β-lactam antibiotics, including penicillin G and ceftriaxone, as well as vancomycin, with minimum inhibitory concentrations (MICs) typically low (e.g., penicillin MIC 0.03-0.06 mg/L for A. urinae).^[41][5] For uncomplicated bacteremia, a 2-week course of intravenous penicillin or ceftriaxone is generally sufficient, while infective endocarditis requires 4-6 weeks of therapy, often extending to 33 days median duration in reported cases.^[41][43] Emerging resistance to fluoroquinolones has been noted in some isolates, particularly A. urinae (up to 50% resistance), limiting their use as alternatives.^[43][5] Treatment guidelines for Aerococcus infections align closely with those for streptococcal endocarditis, prioritizing β-lactams as first-line agents; for severe cases like endocarditis, combination therapy with penicillin and an aminoglycoside is advised due to demonstrated in vitro synergy.^[43][5] Although formal clinical breakpoints are still under development by bodies like EUCAST, susceptibility testing is essential to guide therapy, with vancomycin reserved for β-lactam allergies.^[41][5]

Species

Validly Published Species

As of November 2025, the genus Aerococcus, within the family Aerococcaceae, encompasses 13 validly published species with no synonyms or invalid names recognized.^[4] These species are listed below with their publication details, etymology summaries, and type strain information.

• Aerococcus viridans Williams et al. 1953 (Approved Lists 1980): Etymology derived from the Latin participle viridans, referring to the greening effect on blood agar; type strain ATCC 11563 = CCUG 4311 = DSM 20340.^[44] (the type species of the genus).
• Aerococcus urinae Aguirre and Collins 1992: Etymology from the Latin genitive urinae, of urine, indicating isolation source; type strain ATCC 51268 = CCUG 29291 = DSM 7446.^[45]
• Aerococcus sanguinicola (corrig.) Lawson et al. 2001: Etymology combining Latin sanguis (blood) and -cola (dweller), denoting blood habitat; type strain CCUG 43001 = DSM 15633 = JCM 11549.^[46]
• Aerococcus christensenii Collins et al. 1999: Etymology honoring Danish microbiologist Jens J. Christensen; type strain CCUG 28831 = DSM 15819.^[47]
• Aerococcus urinaehominis Lawson et al. 2001: Etymology from Latin urina (urine), homo (man), and genitive suffix, pertaining to human urine; type strain CCUG 42038 = DSM 15634.^[48]
• Aerococcus suis Vela et al. 2007: Etymology from Latin genitive suis, of a hog, reflecting porcine origin; type strain CCUG 52530 = DSM 21500.^[49]
• Aerococcus urinaeequi (Garvie 1988 ex Mees 1934) Felis et al. 2005: Etymology combining Latin urina (urine) and equus (horse), from equine urine; type strain ATCC 29723 = DSM 20341.^[50]
• Aerococcus vaginalis Tohno et al. 2014: Etymology from Latin vaginalis, pertaining to the vagina, site of isolation; type strain DSM 27293 = JCM 19163.^[51]
• Aerococcus agrisoli Sun et al. 2023: Etymology from Latin ager (field) and solum (soil), of farmland soil; type strain CCTCC AB 2018283 = JCM 33111.^[52]
• Aerococcus loyolae Choi et al. 2023: Etymology honoring Loyola University Chicago, isolation site; type strain ATCC TSD-300 = DSM 115698.^[53]
• Aerococcus mictus Choi et al. 2023: Etymology from Latin participle mictus, of urinating, relating to urine isolation; type strain ATCC TSD-301 = DSM 115699.^[54]
• Aerococcus tenax Choi et al. 2023: Etymology from Latin tenax, tenacious, due to strong biofilm formation; type strain ATCC TSD-302 = DSM 115700.^[55]
• Aerococcus kribbianus Bai et al. 2024: Etymology derived from KRIBB (Korea Research Institute of Bioscience and Biotechnology); type strain JCM 35699 = KCTC 25571.^[56]

Notable Species

Aerococcus urinae is a Gram-positive coccus that typically grows in pairs and clusters, exhibiting alpha-hemolysis on blood agar and testing negative for catalase while negative for pyrrolidonyl arylamidase (PYR) but positive for leucine aminopeptidase.^[15] It is primarily isolated from human urine and is a significant cause of urinary tract infections (UTIs) and urosepsis, particularly in elderly males with underlying comorbidities such as prostate hyperplasia or chronic kidney disease.^[57] The species shows high 16S rRNA gene sequence similarity within the Aerococcus genus, supporting its taxonomic placement.^[58] Aerococcus sanguinicola is associated with bacteremia and infective endocarditis, often in patients with urinary tract abnormalities or prosthetic heart valves, and has emerged as a notable pathogen in clinical settings over the past two decades.^[59] Like other Aerococcus species, it forms Gram-positive cocci in tetrads or clusters and is catalase-negative.^[60] As the type species of the genus, Aerococcus viridans was first isolated from environmental sources such as air and soil, distinguishing it from the more clinically oriented congeners.^[61] It rarely causes human infections, with sporadic reports of endocarditis and UTIs, but is more commonly implicated in veterinary contexts, including porcine and bovine infections.^[62] Phenotypically, it presents as alpha-hemolytic, catalase-negative cocci arranged in pairs, tetrads, or clusters.^[63] Genomes of Aerococcus species, including the notable ones above, typically range from 1.9 to 2.4 Mb in size with G+C contents of 37–47 mol%, reflecting their compact and AT-rich nature.^[64] Unique biochemical traits further differentiate species, such as the hippurate hydrolysis positivity observed in A. christensenii, which aids in laboratory identification.^[15]