European Journal of Cardiovascular Medicine

Staphylococcus lugdunensis is a significant member of the coagulase-negative Staphylococci group and is recognized for its capability to cause severe infections typically associated with Staphylococcus aureus, such as endocarditis, osteomyelitis, skin and soft tissue infections, and catheter-related bacteremia. Despite being coagulase negative, Staphylococcus lugdunensis possesses distinct pathogenic characteristics, including aggressive infection courses and the ability to cause severe clinical outcomes. It was first described in 1988 and since then, has been increasingly implicated in both community acquired and nosocomial infections.1, 2

Accurate identification and differentiation of Staphylococcus lugdunensis from other coagulase-negative Staphylococcus species are essential for effective patient management due to its potential virulence and unique antibiotic susceptibility patterns. Standard laboratory techniques such as the ornithine decarboxylase test and pyrrolidonyl arylamidase activity, along with susceptibility profiling, have become critical tools for proper identification. Timely identification ensures appropriate antibiotic therapy, significantly impacting clinical outcomes. Therefore, continuous surveillance and accurate laboratory identification methods are vital to mitigate the clinical impacts associated with Staphylococcus lugdunensis infections.3,4,5,6

This study was conducted at the Department of Microbiology, Amrita Institute of Medical Sciences, Kerala, India. Ethical approval was obtained from the Institutional Review Board of Amrita Institute of Medical Sciences. A total of 75 clinical isolates initially identified as Coagulase negative Staphylococcus were collected from diverse clinical specimens including Blood, Pus, Urine, and Wound swabs. These samples were screened to identify Staphylococcus lugdunensis based on specific biochemical and microbiological criteria. Identification procedures included ornithine decarboxylase test and pyrrolidonyl arylamidase test. For the ornithine decarboxylase test, isolates were inoculated in decarboxylase broth supplemented with ornithine and incubated aerobically at 37°C for 24 hours. A positive result was indicated by a colour change from yellow to purple. The pyrrolidonyl arylamidase test was performed by inoculating isolates into pyrrolidonyl arylamidase broth, incubating at 37°C for 24 hours, followed by the addition of pyrrolidonyl arylamidase reagent. A positive pyrrolidonyl arylamidase test result was indicated by a pink or red colour formation. Slide and tube coagulase tests were conducted to further differentiate the isolates. The slide coagulase test involved mixing a bacterial colony with a drop of human plasma on a glass slide and observing agglutination within 10 seconds.

The tube coagulase test was performed by incubating isolates with human plasma at 37°C and observing clot formation at intervals of 4 and 24 hours.

Antibiotic susceptibility testing was carried out using the VITEK 2 automated system. Isolates were tested against a panel of antibiotics including beta-lactamase production, cefoxitin, penicillin, oxacillin, erythromycin, clindamycin, gentamicin, ciprofloxacin, vancomycin, and linezolid. Interpretations of susceptibility were based on the guidelines of the Clinical and Laboratory Standards Institute. Quality control measures were maintained using standard strains of Staphylococcus aureus ATCC 25923 and Staphylococcus epidermidis ATCC 12228 for comparison in biochemical tests and antibiotic susceptibility profiles.

Data was analyzed descriptively, presenting the frequency of isolates and their antibiotic susceptibility profiles.

Out of the total 75 Coagulase-negative Staphylococcus isolates screened during the study period, only one isolate (1.33%) was positively identified as Staphylococcus lugdunensis.

This isolate was collected from a pus sample. Detailed biochemical test results showed that the isolate was positive for ornithine decarboxylase and pyrrolidonyl arylamidase. The isolate showed positive slide coagulase reaction but negative tube coagulase reaction, clearly differentiating it from Staphylococcus aureus.

Table 1: Biochemical Characteristics of Staphylococcus lugdunensis isolate

Antibiotic susceptibility testing using the VITEK 2 automated system demonstrated that the Staphylococcus lugdunensis isolate was susceptible to beta-lactam antibiotics, specifically penicillin (MIC 0.12 µg/ml) and oxacillin (MIC ≤0.25 µg/ml). The isolate was also susceptible to aminoglycosides (gentamicin ≤0.5 µg/ml), fluoroquinolones (ciprofloxacin ≤0.5 µg/ml), glycopeptides (vancomycin ≤0.5 µg/ml), and oxazolidinones (linezolid 1 µg/ml).

Table 2: Antibiotic Susceptibility Profile of Staphylococcus lugdunensis Isolate

However, resistance was observed against macrolides (erythromycin ≥8 µg/ml) and lincosamides (clindamycin ≥8 µg/ml). The isolate was negative for beta-lactamase production and cefoxitin screening. Overall, the Staphylococcus lugdunensis isolate displayed a susceptibility profile suggesting that common first-line antibiotics such as beta-lactams remain effective, but emerging resistance to erythromycin and clindamycin warrants consideration in clinical settings.

Staphylococcus lugdunensis is increasingly recognized as a significant pathogen within the coagulase negative Staphylococcus group, capable of causing severe and aggressive infections that often resemble those caused by Staphylococcus aureus. In our study, the isolation rate of Staphylococcus lugdunensis was 1.33%, which is relatively lower compared to other studies where prevalence ranged between 3% to 18%.1,3 This variability in isolation rates can be attributed to differences in geographical locations, population characteristics, and laboratory detection methods.

The biochemical characteristics of isolate, including positive ornithine decarboxylase, pyrrolidonyl arylamidase, positive slide coagulase, and negative tube coagulase, align with standard diagnostic criteria. This ensures the importance of employing multiple biochemical tests to accurately differentiate Staphylococcus lugdunensis from other coagulase negative Staphylococcus species, especially Staphylococcus aureus, due to their overlapping phenotypic characteristics.4,5 Antibiotic susceptibility results revealed high susceptibility of the isolate to beta-lactam antibiotics (penicillin and oxacillin), aminoglycosides (gentamicin), fluoroquinolones (ciprofloxacin), glycopeptides (vancomycin), and oxazolidinones (linezolid). However, resistance was observed against macrolides (erythromycin) and lincosamides (clindamycin). Similar susceptibility patterns have been documented in previous studies, indicating consistent susceptibility to beta-lactams, while emerging resistance patterns to macrolides and lincosamides necessitate ongoing surveillance.6,7

The negative beta-lactamase production and cefoxitin screening indicate that our isolate remains susceptible to commonly used beta-lactam antibiotics, making them viable first-line treatment options. Nevertheless, resistance observed to erythromycin and clindamycin highlights the need for cautious antibiotic selection, advocating for routine susceptibility testing to guide clinical treatment effectively.

Our findings underscore the clinical importance of accurate laboratory identification and susceptibility profiling for Staphylococcus lugdunensis to ensure timely and effective patient management. The current study reiterates that despite its rarity, Staphylococcus lugdunensis should be considered significant, particularly in sterile site infections, and managed appropriately to prevent severe clinical outcomes.

In conclusion, our study emphasizes the significance of accurately identifying Staphylococcus lugdunensis from other coagulase-negative Staphylococcus due to its potential for causing severe infections resembling those by Staphylococcus aureus. The isolate demonstrated high susceptibility to commonly used antibiotics like penicillin, oxacillin, gentamicin, ciprofloxacin, vancomycin, and linezolid, indicating their continued effectiveness for initial therapeutic management. However, observed resistance to erythromycin and clindamycin underscores the necessity for careful antibiotic selection and ongoing surveillance of susceptibility patterns. Regular identification and susceptibility testing are essential to optimize clinical outcomes and effectively manage infections caused by this potentially virulent organism.

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