Urinary Bacterial Isolates
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This is the first of what will be a periodic review of bacterial isolate patterns with antibiotic susceptibility derived from Hunter Area Pathology Service records together with clinical and microbiological commentaries. Two main sources of data have been used to prepare this review:
Two main sources of data have been used to prepare this review:
Hunter Area laboratory data on bacteria isolated from more than 20,000 urine specimens tested through HAPS from 1995 to 2005. Since 1995, data from specimens received from upper Hunter hospitals have been included (Table 1).
Records of all bacteraemic urinary tract infection (UTI) episodes detected from blood cultures submitted to HAPS; clinical data obtained for these episodes at the time of bacteraemia was used to ascribe a site of infection.
Further detail about isolates and bacteraemias from patients in a particular clinical unit is available from HAPS Microbiology.
Commentary on Gram Negative Bacteria
E. coli remains the MOST common urinary isolate accounting for 78% of bacteraemic community-acquired UTI and 31% of nosocomial indwelling catheter-associated UTI (Table 1).
The overall sensitivity of Gram negative urinary isolates has not changed significantly over the period 1992 to 1997 with almost universal sensitivity to gentamicin. Sensitivity to cephalothin, amoxycillin/clavulanate, trimethoprim and nitrofurantoin were all approximately 80% (Table 2, Figure 1).
Pseudomonas aeruginosa overall accounted for 9% of isolates overall and was almost always seen in the nosocomial setting. Gentamicin sensitivity has remained high although declined slightly from 98% to 94% over the period 1992 to 1997. Norfloxacin sensitivity remains high at 93% in 1997.
Other more resistant Gram negative coliforms (eg. Enterobacter cloacae, Serratia and Citrobacter sp) remain a less common cause of urinary infection in the Hunter accounting for 5% of isolates overall in 1997, 2.3% of bacteraemic community UTI and 18% of nosocomial bacteraemic UTI. These isolates remain important because of their innate, chromosomally determined resistance to all b -lactam agents, the expression of which may become clinically apparent in the presence of these drugs.
Commentary on Gram Positive Bacteria
In 1997, Gram positive isolates accounted for 20% of urinary isolates and 5% of bacteraemic UTI, and 18% of community and nosocomial indwelling catheter-related UTI.
Enterococcus faecalis remained the most common isolate in urine and blood. All bacteraemic UTI isolates between 1994-1997 were fully sensitive to penicillin and showed no high-level gentamicin resistance. Limited sensitivity testing of urine enterococcal isolates between 1992 to 1997 showed uniform sensitivity to nitrofurantoin and penicillin (ampicillin/amoxycillin). Isolates of E. faecium, a species of enterococci usually resistant to penicillin remained infrequent (10 in 1996 and 12 in 1997). Two isolates of E. faecium expressing vancomycin resistance (VRE) have been detected in urine over the past two years.
Norfloxacin susceptibility has declined from 81% in 1992 to 41% in 1997. Although, norfloxacin is thought to achieve a therapeutic concentration in urine against enterococci where susceptibility has been demonstrated, it is not recommended for treatment. For this reason, HAPS does not report norfloxacin sensitivity on enterococci.
Isolates of Staphylococcus aureus accounted for 9% of urinary Gram positive isolates. 25 % of 100 isolates in 1997 were Methicillin resistant Staphylococcus aureus (MRSA) with a range of 18 to 31% over the years 1992 to 1996.
Of 102 episodes in 94 patients with S. aureus bacteruria, there were 11 patients (12%) with coexistent bacteraemia detected. Of these 11 patients, 8 had a diverse range of non-urinary primary sites of infection. It therefore remains important to check for coexistent bacteraemia and other possible sites for sepsis in patients demonstrated to have S. aureus in the urine as therapy may require alteration.
Summary
The use of amoxycillin or ampicillin alone as empiric treatment for a suspected urinary tract infection will cover only about 50% of urinary pathogens. Alternative agents such as trimethoprim, amoxycillin/clavulanic acid or cephalexin/cephalothin still cover the majority of pathogens in uncomplicated urinary tract infection.
Amoxycillin or penicillin remain the treatment of choice for the majority of Gram positive urinary pathogens: enterococci, Group B Streptococcus and Staphylococcus saprophyticus. Amoxycillin is the oral antimicrobial of choice for susceptible enterococci.
Commentary on Urine Collection
It is important to ensure that urine specimens are collected properly. The distal female urethra and the perineal area are normally colonised with normal skin, and sometime gut, bacteria, which will contaminate the urine specimen if not collected properly. Similarly, the male urethral orifice is colonised by bacteria. Thus a mid-stream collection is necessary, and the female patient needs instruction to clean the introitus and to hold the labia apart during collection. Please take the time to check that the patient knows how to collect the specimen and ensure that assistance is given where required.
A clean-catch mid-stream urine is the recommended non-invasive method of urine collection from a child. Paediatric bag urines are not recommended as they are invariably contaminated with perineal flora. The amount of contamination is reduced by cleansing the perineum prior to application and not leaving the bag in place for more than 30 minutes. However, if removed immediately after passing urine, and the urine transferred to a urine container, and transported appropriately, bag urines may yield useful results. Invasive methods such as in-out catheterisation or a suprapubic aspirate may be required to clarify findings. It is important to describe the mode of collection on the request form as this may influence the laboratory methods used to examine the specimen.
Table 1: Bacteraemic Urinary Tract Infection
January 1992 - November 1997 |
|
Community-acquired |
Nosocomial (Indwelling catheter-realted) |
Gram negative episodes |
242 (95%) |
74 (80%) |
E. coli |
208 |
33 |
K. pneumoniae |
8 |
5 |
P. aeruginosa |
5 |
13 |
Other |
23 |
23 |
Gram positive episodes |
13 (5%) |
18 (20%) |
Enterococcus spp. |
8 |
6 |
Streptococcus spp. |
2 |
0 |
S. aureus |
1 |
12* |
Coagulase-negative Staphylococcus |
2 |
0 |
* includes three isolates of methicillin-resistant S.aureas (MRSA)
Figure 1: Changes in Gram Negative Antibiotic Susceptibility 1992 to 1997
Reference
10th Edition of Therapeutic Guidelines: Antibiotic (Therapeutic Guidelines Limited, Melbourne, April 1998)
Written by: Drs John Ferguson and Susan Tiley, Microbiology, HAPS
Written: May 1998
Last Reviewed: 10.05.2001