Nitrite-Reducing Bacteria Are Not All Gram-Positive
No, nitrite-reducing bacteria are not exclusively Gram-positive bacteria. Both Gram-positive and Gram-negative bacteria can reduce nitrite, with many important nitrite-reducing species belonging to the Gram-negative category.
Nitrite Reduction in Bacteria: Overview
The ability to reduce nitrite is distributed across both Gram-positive and Gram-negative bacterial species:
Gram-Negative Nitrite-Reducing Bacteria
- According to clinical guidelines, many Gram-negative enteric bacteria are capable of converting dietary nitrates to nitrites in the urinary tract 1
- Common Gram-negative nitrite-reducing bacteria include:
- Escherichia coli
- Klebsiella species
- Pseudomonas species
- Acinetobacter species
- Enterobacter species
- Proteus species
Gram-Positive Nitrite-Reducing Bacteria
- Several Gram-positive bacteria can also reduce nitrite, including:
- Staphylococcus species
- Bacillus species (including Bacillus vireti) 2
- Some Corynebacteria
Clinical Relevance in Urinary Tract Infections
The nitrite test used in urinalysis specifically detects the presence of nitrite-reducing bacteria, which are predominantly Gram-negative:
- The nitrite test in urinalysis is based on the conversion of dietary nitrates to nitrites by bacteria in the urine 1
- This conversion requires approximately 4 hours of bacterial exposure to urinary nitrates 1
- The nitrite test has high specificity (98%) but lower sensitivity (53%) for detecting UTIs 1
- A positive nitrite test strongly suggests the presence of Gram-negative enteric bacteria, which are the predominant nitrite-reducing organisms in UTIs 1
Distribution of Nitrite Reduction Capability
Research evidence shows that nitrite reduction is widespread across bacterial taxonomic groups:
- In a study of nitrate-respiring bacteria from freshwater sediment, both Gram-positive and Gram-negative bacteria demonstrated nitrate and nitrite reduction capabilities 3
- In another study examining bacteria from the achlorhydric stomach, nitrate and nitrite were reduced by both Gram-positive bacteria (staphylococci, corynebacteria, lactobacilli) and Gram-negative bacteria (veillonellae, haemophili, flavobacteria, fusobacteria) 4
- A study of soil and sediment bacteria found that aerobic nitrate respiration (which involves nitrite reduction) was common in Gram-negative genera (Pseudomonas, Aeromonas, and Moraxella) but also present in the Gram-positive genus Arthrobacter 5
Clinical Applications
In urinary tract infections, the nitrite test has important clinical implications:
- A positive nitrite test indicates the presence of nitrite-reducing bacteria, which are predominantly Gram-negative enteric bacteria 1
- In one study, 62.5% of Gram-negative organisms yielded positive nitrite results, compared to only 17.6% of Gram-positive organisms 6
- The nitrite test is highly specific (90-100%) but has lower sensitivity (15-82%) for detecting UTIs 1
- False-negative results are common in infants who empty their bladders frequently, as the conversion of nitrate to nitrite requires approximately 4 hours 1
Conclusion
Nitrite reduction is a metabolic capability found in both Gram-positive and Gram-negative bacteria, though it is more prevalent and efficient in many Gram-negative species, particularly those belonging to the Enterobacteriaceae family. The clinical nitrite test used in urinalysis primarily detects the presence of these Gram-negative nitrite-reducing bacteria, which are the most common causative agents of urinary tract infections.