Recent Records on Bacterial Opportunistic Infections via the Dietary Route

**Background:** Opportunistic infections are serious infections caused by microorganisms that have limited pathogenic capacity under ordinary circumstances but can cause severe disease in individuals with predisposing conditions. This narrative review aimed to identify bacterial opportunistic pathogens that can be transmitted via contaminated food or drinking water and represent a current threat to susceptible patients. The analysis focused on recent case or outbreak reports and systematic reviews published between 2019 and 2023.

**Methods:** A preliminary search was conducted in Google Scholar using the keywords "opportunistic AND pathogen AND human AND food AND infection" limited to the last five years, retrieving 17,500 results. The first 500 items were screened for pathogen names, identifying 42 bacterial genera or species. A second search using individual organism names with keywords "organism name AND food AND human AND infection AND case" or "organism name AND food AND human AND infection AND outbreak" in Google Scholar, PubMed, and Scopus narrowed the list to 24 bacterial groups. Reports were included if they indicated the oral route as the most probable source of infection. A total of 54 recent case or outbreak reports were retained for description.

**Key Results:** The 24 bacterial groups identified include Aeromonas spp., Acinetobacter spp., Arcobacter spp., Bacillus spp., Burkholderia spp., Chryseobacterium spp., coagulase-negative staphylococci (CNS), Comamonas spp., Enterobacter spp., Enterococcus spp., Klebsiella spp., lactobacilli, Lactococcus spp., Laribacter hongkongensis, Leuconostoc spp., Pantoea spp., Pediococcus spp., Plesiomonas shigelloides, Proteus mirabilis, Sarcina ventriculi, Serratia marcescens, Shewanella spp., Streptococcus spp., and Weissella confusa. Key findings include: Aeromonas sobria caused peritonitis in a 37-year-old man on peritoneal dialysis after eating stinky tofu; Arcobacter butzleri caused pericarditis in a 32-year-old HIV-positive man with end-stage renal disease after consuming chicken; Bacillus licheniformis probiotic caused sepsis in two patients with intestinal bleeding; Lactobacillus casei caused endocarditis in a 71-year-old woman taking probiotics; Streptococcus equi subsp. zooepidemicus caused a large outbreak in Italy with 37 cases and 5 deaths linked to unpasteurized milk cheese; Streptococcus suis caused meningitis and hearing loss in a 75-year-old man who ate pork from a sick pig; and Weissella confusa caused endocarditis in a 65-year-old man with cirrhosis who ate sauerkraut. No recent cases of foodborne transmission were reported for Acinetobacter baumannii or Klebsiella pneumoniae, though they are frequently found in food.

**Clinical Implications:** This review highlights that many opportunistic pathogens transmitted via food are not routinely surveilled, leading to underreporting and missed prevention opportunities. Clinicians should consider dietary history in immunocompromised patients with unexplained infections, especially those involving lactic acid bacteria (e.g., lactobacilli, Lactococcus, Leuconostoc, Weissella) or pathogens associated with raw or fermented foods. Probiotic use should be carefully evaluated in patients with compromised intestinal barriers, as translocation can cause bacteremia and endocarditis. Molecular typing methods (e.g., whole genome sequencing) are essential for linking clinical isolates to food sources and for outbreak investigations. Antibiotic resistance profiles were reported in only 15 of 49 reports, underscoring the need for routine susceptibility testing and rapid molecular diagnostics. Public health measures should focus on improving hygiene in food production, especially for raw or fermented products, and educating vulnerable populations about risks associated with raw fish, seafood, unpasteurized milk, and traditional fermented foods.