Ceftriaxone Does NOT Cover Lactobacillus
Lactobacillus species are intrinsically resistant to ceftriaxone and all cephalosporins. This is a fundamental characteristic of the genus and should never be relied upon for coverage 1.
Resistance Profile of Lactobacillus
Lactobacilli demonstrate universal resistance to ceftriaxone and other cephalosporins (ceftazidime, cefepime, cefotaxime, cefazolin, cefoperazone), with resistance decreasing in that specific order but remaining clinically significant across all agents 1.
The resistance pattern extends beyond cephalosporins: Lactobacillus isolates are also resistant to aminoglycosides (amikacin) and fluoroquinolones (ciprofloxacin, norfloxacin) 1.
Lactobacilli carry multiple antibiotic resistance genes, including extended-spectrum β-lactamase genes (blaTEM), vancomycin resistance genes (vanX), and fluoroquinolone resistance genes (parC), which confer broad resistance to β-lactam antibiotics including all generations of cephalosporins 1.
Antibiotics That DO Cover Lactobacillus
Penicillin-type antibiotics are the agents of choice: ampicillin and amoxicillin demonstrate consistent susceptibility against Lactobacillus species 1.
Carbapenems (imipenem, meropenem, ertapenem) show excellent activity against Lactobacillus 1.
Protein synthesis inhibitors maintain activity: chloramphenicol, erythromycin, clarithromycin, and linezolid are effective options 1.
Clinical Context
While ceftriaxone has excellent activity against common gram-negative pathogens (E. coli, Klebsiella, Proteus, Enterobacter) and gram-positive organisms (pneumococci, streptococci, meningococci) 2, 3, Lactobacillus represents a specific gap in its spectrum that cannot be overcome with dose adjustment.
If Lactobacillus coverage is clinically necessary (rare in most infections), ampicillin or a carbapenem must be selected instead of ceftriaxone 1.