June 13, 2018

Relationship Between E coli Growth and Urine pH and Concentration

Using an in vitro system, researchers determined that the bacterium grew more quickly in dilute, acidic urine versus concentrated, alkaline urine.
By Natalie Stilwell, DVM, MS, PhD
Lower urinary tract infection (LUTI) in dogs is caused most commonly by the bacterium Escherichia coli. Conflicting information exists regarding the effect of urine properties on the ability of bacteria to colonize and replicate in the lower urinary tract.

Researchers in New Zealand recently performed a study to examine the effect of urine pH and concentration on in vitro E coli growth.
 

Study Design

Ten apparently healthy, spayed female dogs with a median age of 6.5 years (range, 2-10 years) were included in the study. All dogs had been free of clinical signs associated with LUTI or antibiotic treatment within 3 weeks before the start of the study.

A single, voided, midstream urine sample was collected from each dog into a sterile polypropylene container. Samples were refrigerated for a maximum of 4 hours, filtered, diluted, and titrated to create samples with 9 possible combinations of pH (5.5, 7.0, or 8.5) and urine specific gravity (USG) (1.010, 1.020, or 1.030).

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Three frozen, archived E coli isolates cultured from female dogs with LUTI were incubated at 37ºC and diluted in phosphate-buffered saline to a 0.5 McFarland equivalence turbidity standard. Then, 20 mL of the dilutions were added to aliquots containing 180 mL of urine solution and incubated at 37ºC for 4 hours. Finally, serial dilutions of each sample were incubated on nutrient agar plates and observed for bacterial growth, which was calculated as number of colony-forming units (CFUs) per mL of urine.
 

Results

Voided urine samples had a median pH of 5.8 (range, 5.4-7.2) and median USG of 1.047 (range, 1.037-1.065). Two E coli isolates grew significantly better than the third isolate, although results from all 3 were included in analysis.

The authors found that E coli generally grew fastest at neutral pH (7.0), followed by acidic pH (5.5), then alkaline pH (8.5). However, these differences were statistically significant only at USG values of 1.020 and 1.030, while E coli growth at USG 1.010 was not affected by pH. 

E coli growth was significantly higher in dilute urine (USG 1.010) than in concentrated urine (USG 1.030) for all samples, regardless of pH. Growth was also higher at USG 1.020 than USG 1.030 for 7 of 9 samples.

For context, the authors noted that healthy, adult dogs typically produce slightly acidic urine, although many veterinary diets are formulated to manage urolithiasis by acidifying or alkalinizing the urine.
 

Take-home Message

E coli generally grew better in dilute urine than in concentrated urine. Also, bacterial growth was highest at neutral pH, intermediate at acidic pH, and lowest at alkaline pH. Results suggest that urine properties may affect E coli growth in urine and subsequently influence the risk of LUTI in dogs.

However, the authors cautioned against recommending specific dietary modifications to concentrate and alkalinize the urine for prevention of UTI until future research is performed using an in vivo system.

 
Dr. Stilwell received her DVM from Auburn University, followed by an MS in fisheries and aquatic sciences and a PhD in veterinary medical sciences from the University of Florida. She provides freelance medical writing and aquatic veterinary consulting services through her business, Seastar Communications and Consulting.

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