Laboratory diagnosis of urinary tract infection

Definition

Urinary tract infection (UTI) is the active infection in any part of urinary tract beyond distal urethra which is normally bacteriologically sterile.

Causative Agents

A large number of organisms gaining access to urinary tract are capable of causing UTI (Table 1). These organisms may reach the urinary tract through ascending route or by haematogenous route.

Table 1: Causative agents of UTI

Factors Predisposing to UTI

Various bacterial attributes and host factors favour 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.

Clinical Specimen: Collection and Transportation

Mid-stream sample of urine is the ideal specimen for the diagnosis of UTI. First part of the urine washes away the surface commensals from the distal urethra and hence the midstream specimen indicates actual bacteriological picture of the urinary tract. If it is not possible to collect midstream urine, such as in young children, suprapubic aspiration of urine is done. Catheterized samples are not preferred.

The sample must be immediately cultured, otherwise stored at 4°C. Since urine is an extremely good medium for the growth of bacteria, keeping the sample at room temperature permits the unchecked and unpredictable growth of bacteria and the results obtained thus do not represent the true picture.

For diagnosis of tuberculosis of urinary tract, three consecutive early morning specimens are collected and delivered to laboratory. An alternative is collection of 24 hours’ sample of the urine. For tuberculosis, the urine is centrifuged and deposit is examined for acid fast bacilli.

Microscopic Examination of Urine

A small portion of urine is centrifuged and the deposit is examined for the presence of pus cells, erythrocytes and bacteria. Both pus cells and bacteria are present in urine in UTI. Presence of bacteria in the absence of pus cells is likely to be due to contamination rather than active UTI. Detection of erythrocytes is suggestive of damage or trauma to the urinary tract making it more prone to bacterial infection.

Isolation by 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 loopful. 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 × 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. Bacterial counts in urine are, however, influenced by various factors as shown in table below (table 2).

Table 2: Factors affecting bacterial counts in urine


The colonies obtained on culture media are identified by standard laboratory techniques. The commonest aetiological agent, i.e. Escherichia coli produces pink coloured flat colonies on MacConkey agar. Klebsiellae also produce pink coloured (lactose fermenting) colonies on MacConkey agar which are mucoid.

Identification of Isolates

Wherever possible, detailed biochemical tests should be put up to confirm the identity of the isolate.

Antibiotic Sensitivity Testing

The sensitivity of the isolate is determined to commonly used antimicrobial agents which include nitrofurantoin, ampicillin, sulfonamides, co-trimoxazole and nalidixic acid. Organisms resistant to these drugs are tested for newer antimicrobial agents.



Comments

Post a Comment

Popular posts from this blog

Gram Stain

Image
What is Gram Staining? Gram staining is a common technique used to differentiate two large groups of bacteria based on their different cell wall constituents. The Gram stain procedure distinguishes between Gram positive and Gram negative groups by coloring these cells red or violet. Gram positive bacteria stain violet due to the presence of a thick layer of peptidoglycan in their cell walls, which retains the crystal violet these cells are stained with. Alternatively, Gram negative bacteria stain red, which is attributed to a thinner peptidoglycan wall, which does not retain the crystal violet during the decoloring process. How Does Gram Staining Work? Gram staining involves three processes: staining with a water-soluble dye called crystal violet, decolorization, and counterstaining, usually with safanin. Due to differences in the thickness of a peptidoglycan layer in the cell membrane between Gram positive and Gram negative bacteria, Gram positive bacteria (with a thicker pept...
Read more

Widal Test

Image
  Introduction Widal Test is an agglutination test which detects the presence of serum agglutinins (H and O) in patients serum with typhoid and paratyphoid fever. When facilities for culturing are not available, the Widal test is the reliable and can be of value in the diagnosis of typhoid fevers in endemic areas. It was developed by Georges Ferdinand Widal in 1896 The patient’s serum is tested for O and H antibodies (agglutinins) against the following antigen suspensions (usually stained suspensions): S. Typhi H antigen suspension, d S. Typhi 0 antigen suspension, 9, 12 S. Paratyphi A 0 antigen suspension, 1, 2, 12 S. Paratyphi A H antigen suspension, a S. Paratyphi B 0 antigen suspension, 1, 4, 5, 12 S. Paratyphi B H antigen suspension, b, phase 1 S. Paratyphi C 0 antigen suspension, 6, 7 S. Paratyphi C H antigen suspension, c, phase 1 Salmonella antibody starts appearing in serum at the end of first week and rise sharply during the 3rd week of endemic fever. In acute typhoid fev...
Read more

White blood cells

Image
Granulocytes  Are phagocytes, which have the ability to ingest viruses, bacteria and other parasites. They have visible granules or grains in their cytoplasm and have large elongated or lobed nuclei. The  diameter of cell measures approximately from (12 – 20) μ, and their nucleoli cannot be seen. They account for approximately 60% of our WBCs. The sub-types of granulocytes are: neutrophil, basophil and eosinophil.    Neutrophils   are a part of the innate immune system and an essential line of defense against bacteria. The shape of  nucleus is like a “U” or a curled rod prior to segmentation. They are also known as “band neutrophils”. The diameter usually ranges between (10–18)μ. The cytoplasm is moderate to abundant with a few non-specific granules. Neutrophils account  for  approximately  (1%–3%) of the peripheral WBCs. The diameter of a segmented neutrophil cell  usually ranges between (9–16)μ. They have a multi-lobed nucleus (three o...
Read more

Laboratory Disinfectants.

Image
This post describes the use of disinfectants for routine laboratory decontamination of surfaces and equipment. Definitions:   Disinfectants : Disinfectants are substances that are applied to non-living objects to destroy microorganisms that are living on the objects. Microorganisms : A microorganism or microbe is a microscopic organism that comprises either a single cell (unicellular), cell clusters, or multicellular relatively complex organisms. Antiseptic : Any substance that inhibits the growth and reproduction of microorganisms. Generally includes only those that are used on living objects (as opposed to disinfectants) and aren’t transported by the lymphatic system to destroy bacteria in the body (as opposed to antibiotics ). Decontamination – A process that removes the total burden of all classes of microorganisms, usually using chemicals, heat, and/or pressure.  Disinfection – A process that reduces microbial burden on a surface or object.  Inactivation – The pro...
Read more

Morphological classification of medical important bacteria

Image
Morphological classification of medical important bacteria Morphologically bacteria can resemble: Cocci (Singular: coccus) Bacilli / rods (Singular: rod, bacillus) Vibrios (Singular: vibrio) Spirilla (Singular: spirillum) Spirochaetes (Singular: spirochaet Cocci : These are round or oval bacteria measuring about 0.5–1.0 m in diameter. When multiplying, cocci may form pairs, chains, or irregular groups:   cocci in pairs are called diplococci, e.g. meningococci and gonococci. cocci in chains are called streptococci, e.g. Streptococcus pyogenes. cocci in irregular groups are called staphylococci, e.g. Staphylococcus aureus. Gram reaction : Staphylococci and streptococci are Gram positive, whereas diplococci can be Gram positive or Gram negative. Rods (bacilli): These are stick-like bacteria with rounded, tapered (fusiform), square, or swollen ends. They measure 1–10 µ m in length by 0.3–1.0 µ m in width. The short rods with rounded ends are often called coccobacilli. When mul...
Read more

Autoimmune disease

Image
  What is an autoimmune disease? An autoimmune disease is a condition in which your immune system mistakenly attacks your body tissues and cells as foreign and launches an inappropriate immune response. The immune system normally guards against germs like bacteria and viruses. When it senses these foreign invaders, it sends out an army of fighter cells to attack them. Normally, the immune system can tell the difference between foreign cells and your own cells. In an autoimmune disease, the immune system mistakes part of your body, like your joints or skin, as foreign. It releases proteins called autoantibodies that attack healthy cells. Some autoimmune diseases target only one organ. Type 1 diabetes damages the pancreas. Other diseases, like systemic lupus erythematosus (SLE), affect the whole body. Why does the immune syste attack the body? Doctors don’t know exactly what causes the immune-system misfire. Yet some people are more likely to get an autoimmune disease than others. Ac...
Read more

Coagulase test

Image
Coagulase Test- Principle, Procedure, Types, Result, Uses Coagulase Test Definition A coagulase test is a biochemical test that is used to differentiate Staphylococcus aureus from other Staphylococci species like S. epidermidis and S. saprophyticus on the basis of the ability to produce the coagulase enzyme. The coagulase test is an important test that differentiates the species of the genus Staphylococci into two groups; Coagulase positive Staphylococci and Coagulase Negative Staphylococci. The coagulase enzyme acts as a virulence factor in some organisms as it interacts with the fibrinogen present on the host’s cell surface. Organisms with coagulase usually have a protective barrier around themselves, increasing their pathogenicity and resistance against the immune system. Coagulase is of two types; free coagulase and bound coagulase, each of which is detected by different methods. The bound coagulase is called the clumping factor and is detected rapidly by a slide test. The free c...
Read more