An 89 year old man presented to a community hospital emergency room with weakness and inability to walk. Among other tests, two sets of blood cultures were drawn to rule out sepsis, and submitted to the Microbiology laboratory where they were put into an automated blood culture instrument for incubation. Within twenty four hours both aerobic bottles and one anaerobic bottle flagged as positive. Aerobic bottles were subcultured to Sheep Blood, Chocolate and MacConkey agar plates incubated at 35-37C in 5-10% CO2, as well as an anaerobic Brucella agar plate incubated anaerobically. Anaerobic subculture included a BBE/LKV plate incubated anaerobically. Direct and Day 1 gram stains showed small, non-spore forming gram positive rods. Gram stain of culture Plates were examined on Day 1. A small, whitish, moist colony with a narrow zone of beta hemolysis around and under the colonies was noted on Blood agar plates. Left: blood agar plate; Right: same plate with transmitted light Growth was noted on Chocolate and anaerobic Brucella agar plate. A Bile Esculin slant was inoculated and turned positive the next day. A wet mount was done at the bench and the organism appeared very motile with a characteristic type of "tumbling" motility. The catalase test was positive. A CAMP test was performed; however, results were questionably negative. An automated identification system identified the organism at the 99% level. What is the organism?

Bacteria are simple, microscopic prokaryotes that lack a nucleus. Cell walls are composed of peptidoglycan. The size of the bacteria ranges from 0.3 to 2 um in diameter and 0.5 to 5 um in length. The smaller size of the bacteria leads to a larger surface area-to-volume ratio.As it is given that the gram stain shows a positive result, the bacteria is gram-positive. To detect metabolic functions and the presence of certain enzymes, several bacterial biochemical tests are performed.To detect the presence of the catalase enzyme, a catalase test is used.To assess the ability of bacteria to breakdown tryptophan or utilize citrate as a carbon source, indole tests and citrate utilization tests are performed.MacConkey agar medium is used to detect the fermentation of lactose by bacteria. MacConkey agar medium is a selective medium used to differentiate between lactose-fermenting and lactose non-fermenting bacteria. It is mainly used to detect the fermentation of lactose by bacteria. To differentiate between Gram-positive and Gram-negative bacteria, the medium utilizes bile salts and crystal violet as selective agents that inhibit the Gram-positive bacteria and promote the growth of Gram-negative bacteria. Based on the change in color, the medium differentiates lactose fermenting bacteria from lactose non-fermenting bacteria. When fermentation takes place, the pH of the medium reduces, leading to change in color from pink to red. The medium utilizes peptone as a nitrogen and amino acid source to promote growth of bacteria. Lactose is the carbon source of the medium, using which the bacteria perform their metabolic activities.Blood agar media is used to detect the lysis property of bacteria to cleave red blood cells in the medium. A clear zone around the colonies represents complete hemolysis.Chocolate agar: To grow bacteria that cause infection in respiratory tract.Anaerobic Brucella agar medium is used to detect the presence of anaerobic bacteria in the culture.As the organism is positive for chocolate agar, anarobic Brucella agar, bile esculin slant, catalase test, etc., but negative for CAMP test, and the colonies are whitish, small, moist, and a narrow zone of beta hemolysis is seen around the colonies, the bacteria belong to Streptococcus genus.As the organism is positive for chocolate agar, anarobic Brucella agar, bile esculin slant, catalase test, etc., but negative for CAMP test, and the colonies are whitish, small, moist, and a narrow zone of beta hemolysis is seen around the colonies, the bacteria belong to Streptococcus genus.