All-Pink Gram Stain in a Sputum Sample
A microbiology technician at a district hospital in Madurai receives a purulent sputum sample from a 45-year-old man with fever, productive cough and chest pain. She prepares a Gram stain: a thin smear is heat-fixed, flooded with crystal violet for 1 minute, rinsed, covered with Gram’s iodine for 1 minute, rinsed, decolourised with 95% ethanol for 45 seconds, rinsed, counterstained with safranin for 30 seconds, rinsed, air-dried and examined under oil immersion. All the bacteria, including a known Staphylococcus aureus control slide stained in the same batch, appear pink. The control slide normally shows purple cocci in clusters. The technician says the control is “old” and wants to report the sputum as Gram-negative. The physician is waiting to start empirical antibiotics. The laboratory has fresh reagents, a positive control, and a microscope with a functioning oil-immersion objective.
1. Using the principles of Gram staining, explain the most likely technical error and why the control slide is decisive in this situation.
Read the worked answer
The most likely error is excessive decolourisation. Crystal violet and iodine form a large crystal violet–iodine complex inside the cell. Gram-positive cells have a thick peptidoglycan layer that retains the complex when treated briefly with alcohol, while Gram-negative cells have a thin peptidoglycan layer and a lipid-rich outer membrane that is dissolved by alcohol, so the complex is washed out and the cells take up safranin. If alcohol is left on for 45 seconds, even Gram-positive cells lose the complex and appear pink.
The control is decisive because it is a known Gram-positive organism processed in the same batch. If the control is pink, the stain failed at the decolourisation step or another shared step; the result says nothing reliable about the sputum. The control is a process control, not an old slide to be dismissed. A valid control should show purple cocci in clusters. Without a valid control, the sputum cannot be reported as Gram-negative, and doing so could mislead antibiotic choice.
2. Describe the corrected staining procedure and the expected colours and morphology of Gram-positive and Gram-negative bacteria.
Read the worked answer
A corrected procedure is: prepare a thin, even smear and heat-fix it; flood with crystal violet for 1 minute and rinse; cover with Gram’s iodine for 1 minute and rinse; decolourise with 95% ethanol drop by drop for about 5–15 seconds, stopping as soon as the solvent runs colourless, and immediately rinse with water; counterstain with safranin for 30 seconds, rinse, blot dry and examine under oil immersion. A positive control such as Staphylococcus aureus and a negative control such as Escherichia coli should be stained in the same batch.
The expected result is that Gram-positive bacteria appear purple or violet because they retain the crystal violet–iodine complex. Gram-negative bacteria appear pink or red because they lose the complex and take up safranin. Morphology should also be recorded: Gram-positive cocci may be in clusters, chains or pairs; Gram-negative bacteria are typically rods, cocci or coccobacilli. In a sputum sample, the presence of many polymorphonuclear cells and a predominant organism can guide the clinician, but the Gram stain is only a preliminary guide and must be interpreted with culture.
3. What should be reported to the physician now, and what additional laboratory step would you take before finalising the Gram stain report?
Read the worked answer
The immediate report should state that the Gram stain is unsatisfactory and will be repeated. It should not be reported as Gram-negative. The physician should be informed promptly, because empirical antibiotics may be needed before a valid result is available; the laboratory should not delay clinical treatment but should make clear that the stain is invalid. The specimen quality can still be noted, such as the presence of pus cells, but no organism colour should be finalised.
Before finalising, the technician should repeat the Gram stain with fresh reagents, a properly decolourised smear and a fresh positive control. If the repeat shows a valid control and a predominant organism, the result can be reported with morphology. In parallel, the sputum should be cultured on blood agar and MacConkey agar, and the isolate identified by colony appearance, Gram reaction and biochemical tests. A valid Gram stain showing Gram-positive diplococci may suggest Streptococcus pneumoniae, while Gram-negative rods may suggest Haemophilus, Klebsiella or another organism, but culture and sensitivity remain necessary for a definitive report.