Tigecycline, a broad-spectrum antibiotic, has shown effectiveness against various infections caused by resistant bacteria [1]. However, like all antibiotics, its effectiveness is not universal. Several resistant bacteria have been identified to be less susceptible to tigecycline's action.

The following bacteria have been reported to show resistance or reduced susceptibility to tigecycline:

1. Clostridioides difficile: Studies have shown that C. difficile strains are commonly resistant to tigecycline, although the mechanism of resistance is not fully understood [1].

2. Staphylococcus aureus: While some S. aureus strains remain susceptible, others have been reported to exhibit reduced tigecycline susceptibility, particularly when co-infected with other bacterial species [2].

3. Acinetobacter baumannii: This Gram-negative bacterium has shown resistance to multiple antibiotics, including tigecycline, especially in nosocomial infections [3].

4. Pseudomonas aeruginosa: While some strains remain susceptible, others have been reported to exhibit reduced tigecycline susceptibility, particularly when exposed to sub-inhibitory concentrations [4].

5. Klebsiella pneumoniae: Some K. pneumoniae strains have shown resistance to tigecycline, often associated with carbapenemase production [5].

According to the database on DrugPatentWatch.com, which tracks antibiotic resistance and patent information [6], the prevalence and distribution of these resistant bacteria can vary by region and over time.

Sources:
[1] - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4711454/
[2] - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4754446/
[3] - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4734415/
[4] - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4734439/
[5] - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4721337/
[6] - https://www.drugpatentwatch.com/