What bacteria are covered by Flagyl (metronidazole) and ceftriaxone?

Bacterial Coverage of Flagyl (Metronidazole) and Ceftriaxone

The combination of ceftriaxone and metronidazole provides comprehensive coverage against gram-positive cocci, gram-negative aerobic and facultative bacilli, and anaerobic bacteria, making it an effective regimen for polymicrobial infections, particularly intra-abdominal and pelvic infections. 1, 2

Ceftriaxone Coverage

Gram-Positive Bacteria

• Streptococcus pneumoniae 3
• Staphylococcus aureus (methicillin-sensitive strains) 3
• Streptococcus pyogenes 3
• Viridans group streptococci 3
• Staphylococcus epidermidis 3

Gram-Negative Bacteria

• Escherichia coli 3
• Klebsiella pneumoniae 3, 4
• Proteus mirabilis 3, 4
• Haemophilus influenzae (including beta-lactamase producing strains) 3
• Haemophilus parainfluenzae 3
• Enterobacter aerogenes 3
• Enterobacter cloacae 3
• Serratia marcescens 3
• Neisseria gonorrhoeae (including penicillinase-producing strains) 3
• Neisseria meningitidis 3
• Moraxella catarrhalis 3
• Morganella morganii 3
• Proteus vulgaris 3

Important Limitations of Ceftriaxone

• No activity against anaerobic bacteria - must be combined with metronidazole for infections involving anaerobes 1, 2
• No activity against Pseudomonas aeruginosa (unlike ceftazidime or cefepime) 1
• No reliable activity against Enterococcus species 1
• No activity against atypical organisms (Mycoplasma, Ureaplasma, Chlamydia) 1, 3

Metronidazole (Flagyl) Coverage

Anaerobic Gram-Negative Bacilli

• Bacteroides fragilis (most active agent available) 5, 6, 7
• Bacteroides fragilis group (B. distasonis, B. ovatus, B. thetaiotaomicron, B. vulgatus) 5, 7
• Other Bacteroides species 5, 7
• Fusobacterium species 5, 7

Anaerobic Gram-Positive Bacteria

• Clostridium species (including C. perfringens, but most strains of C. difficile are resistant) 3, 5, 7
• Peptostreptococcus species 3, 5
• Peptococcus niger 5
• Susceptible strains of Eubacterium 5

Other Organisms

• Gardnerella vaginalis 7
• Helicobacter pylori (when used in combination therapy) 7, 8

Protozoal Coverage (Not Bacterial)

• Trichomonas vaginalis 6, 7
• Entamoeba histolytica 5, 7
• Giardia lamblia 7

Critical Limitations of Metronidazole

• No activity against aerobic bacteria (facultative or obligate aerobes) 5, 6
• No activity against gram-negative aerobic bacilli like E. coli, Klebsiella, or Pseudomonas 6
• Variable activity against some anaerobic cocci and nonsporulating gram-positive bacilli 6
• No activity against Propionibacterium 6

Synergistic Coverage of the Combination

When used together, ceftriaxone plus metronidazole provides complementary coverage that addresses the polymicrobial nature of many infections: 1, 2

• Ceftriaxone covers aerobic and facultative gram-positive and gram-negative bacteria 4, 9
• Metronidazole covers anaerobic bacteria that ceftriaxone misses 1, 2
• This combination maintains serum bactericidal titers ≥1:4 against common intra-abdominal pathogens throughout a 24-hour dosing interval 4

Clinical Applications

This combination is specifically recommended for:

• Mild-to-moderate intra-abdominal infections 1, 2
• Necrotizing fasciitis 1
• Pelvic inflammatory disease (though additional coverage for Chlamydia is required) 1, 3
• Incisional surgical site infections of the intestinal or genitourinary tract 1
• Animal and human bites (when anaerobic coverage is needed) 1

Common Pitfalls to Avoid

• Do not use ceftriaxone alone for infections distal to the stomach - anaerobic coverage with metronidazole is essential for appendiceal, colonic, and distal small bowel infections 1, 2
• This combination does NOT cover Enterococcus - empiric enterococcal coverage is generally not necessary for community-acquired intra-abdominal infections 1
• This combination does NOT cover Pseudomonas aeruginosa - consider alternative regimens (piperacillin-tazobactam or cefepime plus metronidazole) for hospital-acquired infections or patients at risk for resistant organisms 1
• Add appropriate antichlamydial coverage (doxycycline or azithromycin) when treating pelvic inflammatory disease, as ceftriaxone and metronidazole have no activity against Chlamydia trachomatis 1, 3
• Consider local resistance patterns - in areas with high ESBL-producing Enterobacteriaceae prevalence, this regimen may not be appropriate for empiric therapy 1