Smallest bacterium amongst the following
(a) Dialister pneumosintes
(b) Nitrosomonas
(c) Bacillus
(d) Spirillum

Hint: It is a non-fermentative, anaerobic, gram-negative rod that grows on blood agar with small, circular, clear, shiny, smooth colonies. In different locations of the body, including the lungs, brain, and dental root canals, pathogenic potential has been shown. It was isolated from other pathogens in humans.

Complete step by step answer:
The nonfermentative, anaerobic, gram-negative bacillus is Dialister pneumosintes. It was formerly called Pneumosintes bacteroides. Among the organisms given, it is the smallest bacterium. Its scale is about $0.5 \times 0.5 \times 1.6 \ \mu m$.
Nitrosomonas is a genus that contains bacteria that are Gram-negative, rod-shaped, and chemoautotrophic. As a metabolic process, this organism oxidizes ammonia into nitrite, known as nitritation (a nitrification step). From the modeling effort, a total of 2,460 protein-encoding genes appeared, averaging 1,011 bp in length, with intergenic regions averaging 117 bp.
Bacillus (Latin 'stick') is a genus of 266 named species of Gram-positive, rod-shaped bacteria, a member of the phylum Firmicutes. The word is often used to describe such bacteria's form (rod) ; and the name of the class of bacteria to which this genus belongs is the plural Bacilli. The typical bacillus is 0.5-1.0 μm wide with a length of 1.0-4.0 μm.
A spirillum (plural spirilla) is a Gram-negative rigid spiral bacterium that also has external amphitrichous or lophotrichous flagella. Examples include the following: members of the Spirillum genus. Campylobacter species, such as the foodborne pathogen that causes campylobacteriosis, Campylobacter jejuni. The biggest spirillum helix, S. Volutans are $5 \ to \ 8 \ \mu m$ (micrometers; $1 \ \mu m = 10^{-6} \ m$) with a length of $60 \ \mu m$.
So, the correct answer is ‘(a) Dialister pneumosintes’.

Note: The infection was believed to be of nasopharyngeal or dental origin in both cases. Following surgical debridement and antibiotic therapy, the patients had positive results. Two strains were graded as D after in vitro amplification and partial sequencing of the 16S rRNA gene.