25/08/2026
What is it? Cocci, non-fermenting bacteria, Escherichia coli?
This is a case reported from a hospital in China. A patient suspected with urinary tract infection was prescribed with urine analysis. Figure 1 is the Gram staining of the specimen, indicating Gram-negative cocci.
After inoculation, the original plate (Fig 2) and the growth of separated colony at 24h (Fig 3) and 48h (Fig 4) all indicated atypical enteric bacilli, resembling non-fermenting bacteria.
Traditional biochemical identification has reached a stalemate—the direction has been wrong.
At this point, MALDI-TOF MS comes into play. It identifies bacteria directly from their protein fingerprints, regardless of morphology or culture characteristics, and quickly provides high-confidence results: Escherichia coli. (Fig 5)
The mass spectrometry results were in complete agreement with the subsequent oxidase test (Fig 6, negative) and MacConkey plate test (Fig 7, pink colonies, fermenting lactose), forming a complete chain of evidence.
Though there was doubt between the different microscopic, inoculation and mass spectrometry results, based on the patient's typical infection symptoms, urinary nitrite level (2+), and history of multiple anti-infective treatments, it was finally speculated as L-form bacteria.
L-form bacteria are variants with partial or complete loss of cell wall, and the common cause is β-lactam antibiotics (this patient had received multiple anti-infective treatments).
In hypertonic urine, Escherichia coli "abandons its cell wall" and transforms into cocci-like L-form in order to survive, thus changing its culture characteristics and deceiving microscopic examination and routine culture.
Verification: The lab technician inoculated the specimens into nutrient-rich liquid medium (blood culture bottles), passaged them, and cultured them under suitable conditions, monitoring them hourly. Ultimately, Escherichia coli recovered its typical rod-shaped morphology and colony characteristics, confirming the accuracy of the original mass spectrometry identification.
📌Key characteristics of L-type bacteria:
✅Hypertonic (must survive in a hypertonic environment)
✅Atavism (can be restored to its original state after the removal of inducing factors, and is divided into variable and stable types).
✅Weak antigenicity (easily evades the immune system, leading to chronic infection)
✅Clinical warning: If a patient has obvious symptoms but a negative routine culture, especially if they have used β-lactam antibiotics, be alert to L-type infection. Special isolation and culture are required, and medication should be adjusted accordingly.
