APPLIED MICROBIOLOGY Urinary tract infections
Definition Urinary tract infection (UTI) is the active infection in any part of urinary tract beyond distal urethra which is normally bacteriologically sterile. Various bacterial attributes and host factors favor urinary tract infections. Mechanical factors are important. Anything that disrupts normal urine flow or complete emptying of the bladder, or facilitates access of organisms to bladder, will predispose an individual to UTI. Adhesion of organism is an important factor in pathogenesis of UTI. The adhesion is mediated by pili or other adhesins such as colonization factor. Once it is adhered, with the help of various virulence factors, it spreads and produces pathogenic effects by resisting bactericidal and bacteriostatic effect of local tissue. Infection may be ascending type or descending type. Causative Agents Infection via ascending route Escherichia coli Klebsiella species Staphylococcus epidermidis Group B streptococci Pseudomonas species Candida species Proteus species Staphylococcus aureus Infection via haematogenous route Salmonella species Mycobacterium species Schistosoma haematobium Histoplasma duboisii Cytomegalovirus Adenovirus type II
Clinical Features 1. Asymptomatic infection or covert bacteriuria is associated with active disease process in kidney. Hence, if left untreated, it can develop cystitis and might go to renal failure. 2. Symptomatic Infection In this, UTI is associated with symptoms as: Cystitis-characterized by dysuria, frequency of micturition, fever, urgency, supra pubic pain and sometimes haematuria. More common in females because of short urethra Acute urethral syndrome-characterized by dysuria and frequency, seen in young, sexually active women Pyelonephritis-characterized by loin pain, tenderness, high fever and rigors. Chronic pyelonephritis causes general ill health and malaise with nocturia Pyelitis-a mild form of pyelonephritis with pyuria but minimal involvement of renal tissue Complications Septicaemia-particularly in elderly Chronic renal failure-due to renal scarring because of persistent or recurrent UTI in the young Clinical Specimen: Collection and Transportation Mid-stream sample of urine is the ideal specimen for the diagnosis of UTI. The sample must be
immediately cultured, otherwise stored at 4°C since urine is an extremely good medium for the growth of bacteria. For diagnosis of tuberculosis of urinary tract, three consecutive early morning specimens are collected. In catheterized patients, sample should be collected directly from the catheter. Laboratory methods A. Microscopy 1. Direct microscopic examination of centrifuged deposits is useful for detection of pus cells, epithelial cells, bacteria, urinary casts, red cells, tubular epithelial cells or atypical cells. 2. Gram stained film may reveal white cells and bacteria that may have come from genital or urinary tract Wherever possible, detailed biochemical tests should be put up to confirm the identity of the isolate. B. Culture Blood agar and MacConkey agar are inoculated with predetermined quantity of urine. This helps in expressing the bacterial count in uniform term of per ml. A standardized loop should be used for inoculation. A loop which delivers 0.05 ml of urine is most convenient. One ml of urine shall contain 200 loopfuls. The number of colonies that are obtained after overnight incubation of inoculated plates is multiplied with 200 to get viable bacterial count per ml of urine. Thus, if the number of colonies on a bacteriological medium is 500, the viable bacterial count per ml of urine shall be 500 x 200 = 100,000. Kass gave a criterion of active bacterial infection of urinary tract according to which a count exceeding 100,000 bacteria per ml denotes significant bacteriuria and is indicative of active UTI. This count is, however, not applicable to tuberculosis of urinary tract because of slow rate of multiplication of mycobacteria. Antimicrobial Susceptibility Testing: Susceptibility testing is essential for choice of antimicrobial agents. Susceptibility is determined by KirbyBauer disc diffusion method using drugs, which are excreted in urine in high concentration. These include: Ampicillin Nalidixic acid Norfloxacin Tobramvcin Augmentin Carbenicillin Cephaloridine Gentamicin Kanamycin Nitrofurantoin Sulphonamides Amoxycillin Cotrimoxazole Cephalexin Ciprofloxacin
The cultures of screened positive urine samples are identified by using standard biochemical and/or serological tests. Treatment: Treatment for urinary tract infections should be guided by susceptibility report as multiple resistances to drugs may occur in uropathogen. Sometimes, a combination of drugs is required.