Non-Lactose Fermenting Bacteria
Non-lactose fermenting bacteria are clinically significant pathogens that include Salmonella, Shigella, Proteus, Pseudomonas aeruginosa, and certain strains of Escherichia coli that lack the ability to ferment lactose sugar, which can cause serious infections with high morbidity and mortality rates. 1, 2
Key Non-Lactose Fermenting Bacteria
Salmonella species: These are consistently non-lactose fermenting and negative in beta-galactosidase (O.N.P.G.) tests, making this characteristic a reliable identification feature for clinical laboratories 1
Shigella species: While some strains like Shigella sonnei and Shigella dysenteriae type 1 are O.N.P.G. positive, they remain non-lactose fermenters on standard media like MacConkey agar 1
Proteus species: These organisms are non-lactose fermenting and typically O.N.P.G. negative, contributing to their identification in clinical specimens 1
Pseudomonas aeruginosa: A significant non-lactose fermenting pathogen that is also O.N.P.G. negative, commonly associated with healthcare-associated infections 1, 3
Non-lactose fermenting E. coli (NLF E. coli): Approximately 10% of E. coli isolates from stool specimens are non-lactose fermenting, and these can be important diarrheagenic pathogens 2, 4
Clinical Significance of Non-Lactose Fermenting Bacteria
Diarrheal Disease: Non-lactose fermenting E. coli can cause significant diarrheal illness, with enteroaggregative E. coli (EAEC) and enteroinvasive E. coli (EIEC) being the predominant pathotypes among NLF E. coli 2
Bloodstream Infections: Non-lactose fermenting gram-negative bacilli cause serious bloodstream infections in immunocompromised patients, including solid organ transplant recipients, with high mortality rates 5
Antimicrobial Resistance: Many non-lactose fermenting bacteria exhibit multi-drug resistance through various mechanisms including β-lactamases, multidrug efflux pumps, and aminoglycoside-modifying enzymes 3, 4
High-Risk Clones: WGS analysis has revealed that non-lactose fermenting E. coli often belong to international high-risk clonal lineages such as ST131 (23%) and ST1193 (18%), similar to their lactose-fermenting counterparts 4
Less Common Non-Lactose Fermenting Pathogens
Stenotrophomonas maltophilia: An emerging opportunistic pathogen with intrinsic resistance to many antibiotics 3
Burkholderia cepacia complex: Important pathogens particularly in cystic fibrosis patients 3
Elizabethkingia species: Opportunistic pathogens with high levels of antimicrobial resistance 3
Chryseobacterium species: Environmental organisms that can cause healthcare-associated infections 3
Achromobacter and Alcaligenes species: Emerging opportunistic pathogens particularly affecting immunocompromised patients 3
Laboratory Identification
MacConkey Agar: Non-lactose fermenting bacteria appear as colorless or transparent colonies on this medium, distinguishing them from lactose fermenters which appear pink 1, 2
Beta-galactosidase (O.N.P.G.) Test: This test detects the enzyme that breaks down lactose and can help differentiate between true non-lactose fermenters and late lactose fermenters 1
Molecular Methods: Multiplex PCR and whole-genome sequencing are increasingly used to identify and characterize non-lactose fermenting bacteria, particularly for pathotyping diarrheagenic E. coli 2, 4
Clinical Implications
Diagnostic Considerations: Non-lactose fermenting colonies on MacConkey agar should not be dismissed, as they may represent significant pathogens including NLF E. coli, Salmonella, or Shigella 1, 2
Treatment Challenges: Many non-lactose fermenting bacteria exhibit multi-drug resistance, with some NLF E. coli showing resistance to ciprofloxacin (82.5%), ampicillin (77.8%), and cotrimoxazole (68.2%) 2, 3
Prevention Strategies: Given the high mortality associated with infections caused by non-lactose fermenting bacteria, prevention is crucial, particularly in immunocompromised patients 5