This study revealed that follow-up gram-stain can be used to predict antibiotic resistance. Furthermore, it was suggested that the possibility of antibiotic resistance is strong when the elongation of gram-negative rods is observed. A number of studies evaluated the usefulness of urinary gram-stain in urinary tract infection. For example, sensitivity and specificity of urinary gram-stain were reported to be 91% (95% CI: 0.86-0.96) and 96% (95% CI: 0.92-0.98) in urinary tract infection of children, respectively, being more favorable than urinalysis and urine sediment examination[3]. Furthermore, a study reported that urinary gram-stain leads to the appropriate choice of antibiotics for the initial treatment of urinary tract infection[4]. However, most previous studies evaluated the usefulness as a diagnostic adjunct, and as far as we searched, there has been no study evaluating the usefulness of gram-stain to predict the effectiveness of antibiotic treatment.
Clinical symptoms such as fever, chills, nausea, and lateral abdominal pain improve within 3 days after appropriate treatment in most cases. On the other hand, 26 and 13% of patients still have an unresolved fever on treatment days 2 and 3, respectively[5]. Therefore, the effect of antibiotics cannot be accurately predicted until the 3rd day based solely on the clinical course. Furthermore, physicians must wait to obtain the results of urine culture and antibiotic sensitivity tests, and so follow-up with gram-stain, which can be promptly evaluated, is of marked benefit.
This study suggested that switching to broader-spectrum antibiotics should be considered when the bacterial cells remain on follow-up gram-stain. Especially when elongation is observed, bacteria are more likely to be resistant to antibiotics, and it is a strong basis for a switch. Evaluation of antibiotic effectiveness at an early stage can lead to an improved treatment outcome. In addition, it becomes easier to use narrow-spectrum antibiotics from the initial treatment, and it may encourage switching to oral agents and early discharge, contributing to reductions in both antimicrobial resistance and medical expenditure. On the other hand, the disappearance of cells on follow-up gram-stain should not be the sole basis for effectiveness of antibiotics. It is necessary to evaluate the effectiveness in reference to the clinical course, as conducted conventionally, until the results of urine culture are obtained.
The low sensitivity of follow-up gram-stain for detecting resistant bacteria may be associated with the number of fields of view or timing of follow-up. In this study, the time spent observing and number of such fields were not set, and observation may have been considered sufficient after only a few fields of view in patients with a favorable clinical course. Furthermore, the follow-up timing was set as within 48 hours, and long-term exposure to the antibiotics may have eliminated resistant bacteria. Earlier follow-up may have led to the detection of remaining resistant bacteria.
It was reported that E. coli cells undergo gradual elongation, and lysis occurs after approximately 70 minutes when exposed to antibiotics they are sensitive to[6]. Therefore, the presence of elongated cells may be considered to suggest that antibiotics are effective. However, it was also reported that a similar change is observed when cefotaxime-resistant E. coli is exposed to cefotaxime[7]. Although elongation itself cannot predict sensitivity or resistance to antibiotics, the continued presence of elongated bacteria after long-term exposure to antibiotics may suggest resistance. In this study, follow-up gram-stain was performed on the day after admission in most cases, following a long period of exposure to antibiotics. As a result, it was considered that the presence of elongated cells at follow-up was associated with a high rate of resistant bacteria.
Limitations of this study include the following. First, as mentioned above, the number of fields of view and amount of time for observation were not set. The favorability of the clinical course may have influenced the observation time and number of fields of view. Secondly, the extraction of cases was based on the final diagnosis on the discharge summary, and patients with disorders such as obstructive pyelonephritis and renal abscess were included. Third, backgrounds of patients were not investigated, and those with urinary retention and urinary catheter insertion were included, possibly generating differences in the degree of drainage of bacteria and urine among cases. Lastly, the timing of follow-up gram-stain was not set. In most cases, it was performed on the day after admission, but the amount of time until follow-up differed depending on the time of admission. The diagnostic accuracy may increase if appropriate follow-up timing is set.