Final answer:
The bacteria most likely to be Bacillus cereus, as it forms white colonies on XLD agar and is consistent with being a gram-positive bacillus which does not ferment lactose on MacConkey Agar.
Step-by-step explanation:
Based on the various pieces of information provided, the bacteria in question, which shows white colonies on Xylose Lysine Deoxycholate (XLD) agar, could potentially be Bacillus cereus. This is deduced from the image that shows white colonies of B. cereus on sheep blood agar. Additionally, other common foodborne pathogens like E. coli, Salmonella spp., and Shigella spp. were ruled out, as their growth on the XLD and MacConkey agars did not exhibit the expected colony colors for these organisms.
Other confirmatory tests, such as the Gram stain, show Bacillus cereus as gram-positive bacilli, and catalase test results are typically positive for this organism. Given that the question does not provide information about colony appearance on MacConkey Agar, but one can infer from the information that the organism is likely a non-lactose fermenter if it yielded white colonies on XLD agar, which aligns with the expected results for Bacillus cereus.