Medical Lab Technology

Dientamoeba fragilis trophozoites, identified via trichrome stain, are small (5–15 µm) amoeba-like flagellates typically containing two nuclei with fragmented karyosomes (3–5 granules). They exhibit a grayish-blue or green-tinted cytoplasm with fine, granular, or vacuolated texture and possess broad-lobed, transparent pseudopodia, making them a, key, frequently, overlooked, cause, of, intestinal, disease.

Because D. fragilis has no known cyst stage and is fragile, permanent stains like trichrome are essential for diagnosis. Direct wet mounts are frequently inaccurate.

Because trophozoites are fragile, stool samples must be immediately fixed (e.g., in PVA) to preserve the characteristic nuclear structure.

The pathogenicity and clinical importance of D. fragilis continue to be investigated, including whether particular genotypes, subtypes, or strains of D. fragilis are associated with symptomatic infection in humans. Both asymptomatic and symptomatic infection (e.g., with various nonspecific gastrointestinal symptoms) have been reported. The reported clinical manifestations have sometimes been described as similar to those of colitis, appendicitis, or irritable bowel syndrome.

Egg of H. nana in a wet mount stained with iodine.

Hymenolepiasis is caused by two cestodes (tapeworm) species, Hymenolepis nana (the dwarf tapeworm, adults measuring 15 to 40 mm in length) and Hymenolepis diminuta (rat tapeworm, adults measuring 20 to 60 cm in length). Hymenolepis diminuta is a cestode of rodents infrequently seen in humans and frequently found in rodents.

Eggs of Hymenolepis nana are immediately infective when passed with the stool and cannot survive more than 10 days in the external environment

Hymenolepis nana is the most common cause of all cestode infections, and is encountered worldwide. In temperate areas its incidence is higher in children and institutionalized groups

Hymenolepis nana and H. diminuta infections are most often asymptomatic. Heavy infections with H. nana can cause weakness, headaches, anorexia, abdominal pain, and diarrhea.

H. nana eggs are oval and smaller than those of H. diminuta, with a size range of 30 to 50 µm. On the inner membrane are two poles, from which 4-8 polar filaments spread out between the two membranes. The oncosphere has six hooks.

Hymenolepis nana (preferred name now Rodentolepis nana) is common in humans because of multiple routes of infection. Humans can be infected by ingesting eggs from mouse or human faecal contamination, or by ingesting insects which are intermediate hosts.

Picture courtesy Anam Mustafa

Proteus swarming on a blood agar plate

Proteus is a member of the order Enterobacterales, comprising Gram-negative, facultatively anaerobic bacilli capable of surviving in diverse and nutrient-limited environments. A defining characteristic of Proteus species is their remarkable motility, mediated by numerous peritrichous flagella.

One of the most striking laboratory features of Proteus is the swarming motility, observed as concentric waves of spreading growth across solid agar surfaces, producing the classic “bull’s-eye” or terraced colony pattern on agar plates.

Swarming represents a highly organized, multicellular migration phenomenon driven by cyclic biological differentiation:

🌐 Short vegetative (“swimmer”) cells differentiate into elongated, multinucleate, hyper-flagellated swarmer cells
🌐 Swarmer cells migrate collectively across solid surfaces in coordinated groups or “rafts”.
🌐 Migration is followed by dedifferentiation back into short vegetative cells.
🌐 Cellular multiplication occurs during a resting consolidation phase.
🌐 The cycle then repeats, allowing progressive colony expansion.

Each migration cycle produces a visible growth terrace, resulting in the characteristic concentric ring appearance.

This synchronized population movement represents a form of bacterial social behavior, regulated through environmental sensing and cell-to-cell signaling mechanisms. Surface contact — particularly inhibition of flagellar rotation upon encountering solid media — triggers expression of swarming-associated genes.

The swarmer cell state is not merely a locomotion strategy but is closely linked with virulence:

🌐 Increased production of urease, leading to alkaline urine and struvite stone formation
🌐 Enhanced expression of hemolysin, contributing to tissue damage
🌐 Rapid surface colonization, especially of urinary catheters
Ability to migrate against urine flow, promoting ascending catheter-associated urinary tract infections (CAUTIs)
🌐 Heavy growth may also produce a characteristic fishy or burnt-chocolate odor in culture.


Swarming growth may obscure isolation of other pathogens in polymicrobial specimens.

To suppress swarming and enable colony separation, inhibitory media may be used:

🌐 MacConkey agar
🌐 Deoxycholate citrate agar (DCA) — bile salts inhibit swarming
🌐 CLED agar — electrolyte deficiency prevents swarm differentiation

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