The genus Aerococcus comprises bacteria increasingly recognized as human pathogens, though they are often initially overlooked in clinical microbiology laboratories. These organisms can be easily confused with other common bacteria, such as Streptococcus or Enterococcus, leading to potential misdiagnosis or delayed treatment. Modern identification techniques have revealed the growing clinical significance of Aerococcus species in causing serious infections. This article explores the nature of these bacteria, the specialized procedures needed for their correct identification, and the therapeutic strategies required to manage the infections they cause.
Characteristics of the Bacteria and Clinical Presentations
Aerococcus species are Gram-positive cocci that typically arrange themselves in pairs, tetrads, or small clusters. They are non-motile and catalase-negative, a characteristic that often causes them to be initially mistaken for members of the Streptococcus group in routine laboratory work. These organisms are considered part of the natural flora in the environment and are sometimes found on the skin or in the human urinary tract.
Aerococcus urinae and Aerococcus viridans are the most frequently identified causes of human disease, alongside Aerococcus sanguinicola. A. urinae is the most common species isolated from clinical samples and is strongly associated with complicated urinary tract infections (UTIs). Infections caused by A. urinae frequently affect elderly individuals, particularly men with underlying urological conditions or those with indwelling urinary catheters.
While A. urinae primarily causes UTIs, it can also lead to invasive conditions such as bacteremia and infective endocarditis (infection of the heart valves). A. viridans is more often linked to severe systemic infections, including endocarditis, septicemia, and occasionally meningitis, particularly affecting immunocompromised or elderly individuals. Both species are recognized for their potential to cause life-threatening invasive disease.
Laboratory Identification Procedures
The correct diagnosis of an Aerococcus infection presents a challenge because the bacteria share many phenotypic characteristics with other common organisms. Initial laboratory work involves culturing specimens collected from the suspected site of infection, such as urine or blood, which reveal alpha-hemolytic, Gram-positive cocci. The arrangement of the bacteria in tetrads or clusters on the Gram stain provides a first clue, differentiating them from the chain-like formations typical of many streptococci.
Definitive identification requires specialized biochemical testing to distinguish Aerococcus from other genera. The pyrrolidonyl arylamidase (PYR) test is useful: A. viridans typically yields a positive result, similar to Enterococcus species, but A. urinae is characteristically PYR-negative. Distinguishing between these two common pathogenic species can also be achieved using the Leucine aminopeptidase (LAP) test, which is typically positive for A. urinae.
Advanced molecular and mass spectrometry techniques have significantly improved diagnostic accuracy. Matrix-Assisted Laser Desorption Ionization-Time of Flight Mass Spectrometry (MALDI-TOF MS) is now widely used in clinical laboratories and has led to a notable increase in the correct identification rate of Aerococcus species. For cases where commercial systems struggle to provide species-level identification, 16S rRNA gene sequencing remains the gold standard for genotypic confirmation.
Standard Treatment Protocols
The management of Aerococcus infections is complicated by the organism’s inherent resistance profile, necessitating careful selection of antimicrobial agents. Aerococcus species possess intrinsic resistance to certain common antibiotics, including sulfonamides (such as trimethoprim-sulfamethoxazole) and may show reduced susceptibility to several cephalosporins and fluoroquinolones. This intrinsic resistance means that empirical treatments often used for common infections may be ineffective.
First-line treatment for most serious Aerococcus infections involves beta-lactam antibiotics, to which the organisms are generally susceptible. Penicillin G or ampicillin are frequently the preferred agents and are typically effective even at standard doses. For severe systemic infections, such as endocarditis or septicemia, combination therapy is often employed to achieve synergistic killing. This usually consists of a beta-lactam agent combined with an aminoglycoside, such as gentamicin, for a limited duration.
The duration of antibiotic therapy must be tailored to the site and severity of the infection. A straightforward urinary tract infection may require a shorter course, while invasive infections like endocarditis necessitate prolonged treatment. Treatment for infective endocarditis commonly spans four to six weeks of intravenous antibiotics to ensure complete eradication of the infection.
Antibiotic susceptibility testing (AST) remains crucial in guiding treatment, especially in cases of severe disease, treatment failure, or known patient allergies. While Aerococcus species are generally susceptible to vancomycin, it is often reserved as an alternative for patients with a known penicillin allergy. Clinicians must rely on AST results to confirm the susceptibility of the isolated strain and select the most appropriate therapeutic regimen.