'School of fish' appearance under microscope is seen in:

The causative agent of chancroid / soft sore is H. ducreyi. When examined by Gram stain, organisms from culture often clump in long parallel strands, producing a so-called "school of fish" or "railroad track" appearance. This morphology can occasionally be seen in Gram-stained smears from clinical specimens Chancroid is a genital ulcerative disease (GUD) and sexually transmitted infection characterized by soft (nonindurated) ulcers with irregular borders and tender inguinal lymphadenopathy, or buboes. The specimen of choice for the diagnosis is a swab that has been taken from the base of the genital ulcer. Culture is considered the 'gold standard'. H ducreyi can grow well on a chocolate Agar supplemented with 1% Iso VitaleX and 5% sheep blood or fetal calf serum. High levels of carbon dioxide are preferred. However, nucleic acid amplification methods are more sensitive.