Multisite Colonization Ecology of Multidrug-Resistant and Commensal Bacteria Among Cardiovascular Outpatients: A Cross-Sectional Study from Cairo, Egypt

Background/Objectives: Bacterial colonization is an under-recognized component of antimicrobial resistance (AMR), particularly among cardiovascular outpatients who frequently interact with healthcare services. While surveillance has traditionally focused on multidrug-resistant (MDR) pathogens causing clinical infection, the ecological characteristics of colonization including multisite carriage, polymicrobial communities, and commensal or low-virulence organisms recovered on antimicrobial-containing media remain poorly defined. This study investigated the burden and ecological patterns of colonization by organisms with reduced antimicrobial susceptibility in cardiovascular outpatients. Methods: In this cross-sectional study, 36 adult cardiovascular outpatients were screened using sputum, nasal, and skin specimens (108 samples) on antimicrobial-supplemented selective media. Isolates were identified by MALDI-TOF mass spectrometry, susceptibility was determined using the VITEK 2 system, and colonization patterns were analyzed using exact statistical methods. Results: Colonization by organisms with reduced antimicrobial susceptibility was detected in 61.1% of patients (95% CI, 44.9–75.2), and at least one validated MDR pathogen in 52.8% (95% CI, 37.0–68.0). Sputum showed the highest colonization prevalence. Multisite colonization occurred in 72.7% of colonized patients, with 40.9% positive at all three anatomical sites. Polymicrobial colonization was common (mean 1.9 species per colonized patient). Resistant commensal bacteria including Neisseria flavescens, Neisseria mucosa, Moraxella catarrhalis, and Lacticaseibacillus casei were recovered alongside recognized pathogens and showed reduced susceptibility across multiple antibiotic classes. Colonization showed significant concordance across anatomical sites, indicating a whole-body colonization state rather than a series of independent site events. Prior hospitalization and concurrent antibiotic therapy were significantly associated with colonization, whereas prior percutaneous coronary intervention and individual comorbidities were not. Conclusions: Cardiovascular outpatients frequently harbour complex bacterial communities characterized by multisite colonization, polymicrobial carriage, commensal and low-virulence organisms recovered on selective media, and MDR pathogens. These findings highlight bacterial colonization as a potential community reservoir of AMR and support targeted pre-procedural screening together with community antimicrobial stewardship. As a single-centre, cross-sectional study of 36 patients, these results are exploratory and hypothesis-generating and require confirmation in larger, multicentre cohorts.