Question

Answer the question s below: A 45-year-old homeless man who abuses alcohol presents to the emergency department with fever and cough of 4-day duration. The cough is productive with thick, bloody phlegm. He complains of pain in the right side of his chest with coughing or taking a deep breath. He denies any other medical history and says he cannot remember the last time he saw a doctor. He does not smoke cigarettes but says he drinks a pint of whiskey whenever he can get it, and denies drug use.

His temperature is 38.9oC, pulse 105 BPM, and respiratory rate is 30/breaths/minute. The lung examination is notable for decreased breath sounds and crackles in the right, lower posterior field. His WBC count is elevated. Chest radiography reveals a dense right lower lobe infiltrate with evidence of pulmonary abscess. Sputum samples are collected for gram stain and culture.

The gram stain revealed gram negative rods with numerous white blood cells. Large, very mucoid colonies were found on BA and MacKonkey plates. Pink colonies were seen on the MacKonkey plate.

1. What is your preliminary diagnosis of the organism?
2. What does the pink in the colony on the MacKonkey plate signify?
3. What does a very mucoid colony reveal about an organism?
4. The patient was initially given ampicillin however the organism was resistant to ampicillin. What makes the organism resistant to ampicillin? What other antibiotic would be effective?

Answer 1

The preliminary diagnosis of the organism suggests that the organism is a gram negative bacteria. Also the symptoms of the patient, indicate that the bacteria is likely to be Pseudomonas aerugonisa.

The bacteria, which are capable of fermenting lactose, create acidic waste products. These acidic waste products of bacteria cause pH sensitive dye on MacConkey plate to turn pink. Hence, the gram negative bacteria, which ferment lactose, grow in pink colonies.

A very mucoid colony refers to a very sticky and wet colony. Such colony is mainly formed, when the organism is involved in the process of producing a capsule from the lactose, present in the agar.

Ampicillin inhibits the activity of bacterial enzyme, called transpeptidase. Transpeptidase enzyme helps the bacteria to form the cell wall. This enzyme is present in the peptidoglycan layer of gram positive bacteria. Peptidoglycan layer is not covered in gram positive bacteria. However, Ampicillin is ineffective against gram negative bacteria, because the peptidoglycan layer of gram negative bacteria is covered by outer lipid membrane layer. This outer lipid layer, blocks the ampicillin from reaching to the peptidoglycan layer.

Aminoglycocide is highly effective against gram negative bacteria, because this antibiotic is able to cross the outer lipid layer of gram negative bacteria.