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Sensitivity Testing and Antibiotics

Posted Aug 27 2008 8:33am

Sputum cultures are done frequently for CF patients. A sputum sample is obtained by coughing phlegm into a specimen cup. The patient’s name, the current date and time that the sample was given are recorded on cup’s label. The doctor will then fill out a lab slip, which indicates which tests are to be performed on the sample.


The first round of testing merely determines whether bacteria are present in the sample. A small amount of the sputum is spread across a Petri dish that is filled with a substance called agar gel. Agar gel is essentially “food” for bacteria. If in fact there are indeed microorganisms present in the sputum sample, then they will grow rapidly in the Petri dish thanks to all the food available to them. Sometimes the appearance of bacteria growth is seen within 24 hours, other times it may take a week for any growth to appear. When the lab technician observes bacterial growth, the sample is said to be “positive.”


People with CF will almost always have a positive sample. It is important not only to verify that bacteria or fungus are present in the sample, but which species. In order to identify the microorganism(s) in the sputum, the lab is instructed to carry out something called “sensitivity testing.”


Gram Stains


Before sensitivity testing can be done, the types of bacteria present in the sample need to be determined. This involves a process called “gram staining.” Gram staining (which was invented by Hans Gram in 1884) involves applying a tincture to a collection of the bacteria sample on a microscope plate.

The structure of the cell wall of the bacteria being analyzed determines whether it’s a gram negative or gram positive sample. Bacteria that are gram positive have a compound in their cell walls called peptidoglycan which will turn dark purple when the stain is applied.

Bacteria without peptidoglycan in their cell walls are said to be gram-negative. This is because their appearance will remain unchanged when the stain is applied. Gram-negative bacteria generally have a secondary membrane layer and lipopolysaccharide which act as a protective covering around the cell, preventing the stain from penetrating it.

Sensitivity Testing

Once a bacterium has been identified as either gram positive or gram negative, sensitivity testing is then done to see which type of antibiotics will kill the infection. Lab tests have been streamlined in order to find the most common solutions first. There are different classes of antibiotics that work on any number of infections. The following list shows the classes of bacteria, the class of antibiotic that will target it, and examples of the species of bacteria that may be present in the sample.

Gram negative bacteria

-Pseudomonas aeruginosa

-Cepacia

-Legionella

-Haemophilus influenzae (ear, lung infections)

-Salmonella

-Shigella

-Klebsiella (pneumonia)

-E.Coli (diarrhea)

Sensitivities may indicate using: Amoxycillin, Augmentin, Erythromycin, Gentamicin, Kanamycin, Meropenem, Tobramycin, Cipro, Azithromycin

Pharmacologists have observed that in treating gram-negative bacteria, combination therapy (i.e. prescribing multiple antibiotics at once) is more effective than using just one. The synergistic effects that result from the combination of drugs has been successfully demonstrated.

Gram-positive

-Staph aureus (Toxic Shock Syndrome)

-Streptoccocus

-Listeria

-Bacillus anthracis (Anthrax)

-TB

-Botulism

Sensitivities may indicate using: Kflex, Cephalexin, Augmentin, Amoxicillin, Vancomycin, Cipro

When CF patients experience a pulmonary exacerbation (i.e. worsening of symptoms) the culprit is usually a bacterium that has been cultured before. In order to save time and get the patient started on a course of antibiotics as soon as possible, the doctor may prescribe a drug that has widespread use. Ciprofloxacin for example, is a broad-spectrum antibiotic that is effective against many infections that are common to CF patients. It is used to treat gram-negative and gram-positive bacteria. Once the results of the sensitivity testing comes back, the doctor may then prescribe a different antibiotic that will target the identified infection.

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