Antibiotic Susceptibility of MALDI–TOF Identified Airborne Bacterial Isolates Cultivated from Sofia Urban Air

The microbiological quality of air in cities is crucial for the citizens’ health. The present study aimed to assess the air’s microbiological quality in the capital of Bulgaria, Sofia. Five densely urbanized and highly car-traffic-intensive city points were monitored, and 10 bacterial isolates were selected for antibiotic susceptibility testing. The isolates were identified using the MALDI-TOF approach as the Gram-positive Bacillus cereus and Bacillus thuringiensis. Antibiotic susceptibility was assessed using the agar diffusion method, as outlined in EUCAST 2023, and performed on Mueller-Hinton agar plates. Six antibiotics – ampi¬cillin, amikacin, tigecycline, ciprofloxacin, linezolid, and erythromycin were included in the screening. Our results showed moderate microbial contamination at the examined points in Sofia city. The antibiograms demonstrated susceptibility of all bacterial air isolates to five out of six antibiotics - amikacin, tigecycline, ciprofloxacin, linezolid, and erythromycin. All 10 strains of B. cereus and B. thuringiensis were resistant to the β-lactam ampicillin, and only one strain was resistant to the macrolide erythromycin. In comparison to findings from other researchers, the air in Sofia shows moderate contamination by microorganisms. Ele¬vated levels of microbes in the air pose significant risks, especially for individuals with chronic respiratory conditions, those who are immunocompromised, pregnant women, and children.