Gram-negative bacteria are bacteria with thin, single-layered cell walls that cannot retain the crystal violet color due to no or very weak interaction with gram’s stain, and hence appear as light red or pink when observed under the microscope.
Pseudomonas, Klebsiella, Proteus, Salmonella, Providencia, Escherichia, Morganella, Citrobacter, etc. are some of the examples of gram-negative bacteria.