18: Throat and Urine Cultures

INTRODUCTION

Health care providers will often be expected to obtain samples from patients to diagnose a particular infectious disease. When doing this, it is important to use aseptic technique and proper precautions to protect the person handling the sample, avoid contamination of the surrounding area, and preserve the integrity of the sample. Following proper aseptic technique guarantees that what grows on the Petri dish came from the patient’s sample and is not an environmental contaminant. There are numerous clinical tests that can be used to identify specific types of infections—you will use two of these in this laboratory activity.

Throat Cultures 

Health care providers will often take a throat culture from a patient complaining of a sore throat. This is most commonly used to diagnose strep throat, which is caused by Streptococcus pyogenes. Although there are now “rapid-strep” tests that may be used, a more traditional way of identifying this organism in a throat culture is to use Blood agar. This media contains 5% sheep’s blood, and allows for the distinction between types of hemolysis. Many bacterial species are α- hemolytic, which means they can partially break down red blood cells. Bacteria that are β- hemolytic can completely break down red blood cells; those that are γ-hemolytic do not break down red blood cells at all. Streptococcus pyogenes is a β-hemolytic organism— colonies of this species will completely lyse red blood cells, and so clear zones will be seen around the colonies on a blood agar plate (BAP). Other species, such as Streptococcus pneumoniae, are α- hemolytic, meaning that they can partially break down red blood cells—these colonies will have a greenish pigment around them on a BAP. Species that are non-hemolytic (γ-hemolytic) will grow on Blood agar, but no color change will be observed (e.g. Enterococcus faecalis, Streptococcus salivarius)

The diagnosis of strep throat is important because if left untreated, it may progress to scarlet fever and/or a more serious infection that can result in cardiac tissue damage. A positive strep test indicates the need for antibiotic therapy.

Streptococcus pyogenes infections are not confined to the respiratory tract. This organism can also cause skin infections such as erysipelas, cellulitis and impetigo. In rare cases, some strains of S. pyogenes can invade and multiply in the fascia, causing necrotizing fasciitis, resulting in rapid tissue destruction with mortality rates that can exceed 40%.

Urine cultures 

Urine cultures are taken to diagnose urinary tract infections (UTIs), a very common type of nosocomial infection (an infection acquired during a stay at a hospital or chronic care facility). Many types of bacteria can cause UTIs but E. coli is the most common culprit. Although urine within the body is generally microbe-free, some bacteria are picked up as urine leaves the body. Therefore UTIs are generally diagnosed by determining the amount of bacteria present—low numbers are not usually indicative of an infection unless the organism detected is a very serious pathogen that would not be part of the normal microbiota. To minimize the amount of bacteria from the external genitalia that are present in a urine sample, patients are usually asked to wipe the area before urinating, and a “clean catch” is taken by letting some urine go before collecting—this flushes away the external normal microbiota. A urine sample will often appear cloudy if an infection is present. Selective and differential media, like EMB and BA, can be used to determine the types of bacteria that are present in a urine samples. E. coli can be identified on EMB by the green metallic sheen of the colonies. Generally, more than 100,000/ml of one type of organism reflects significant bacteriuria (bacteria in the urine), but lower numbers of certain pathogens may also indicate infection. Types of UTIs include urethritis (infection of the urethra), cystitis (bladder infection) and pyelonephritis (kidney infection).