Table 1.

Medically important bacterial species, their resistance mechanisms, therapeutics where resistance is evident, and associated morbidity.

Bacterial Species	Resistance Mechanisms	Therapeutics	Morbidity
Gram-negative
Klebsiella pneumoniae *	Mutations in chromosomal genes, horizontal gene transfer (HGT) [20], efflux pump, ESBL production, intrinsic resistance [21] Reduced access to bacterial targets [22]	Carbapenem, third-generation cephalosporin [2]	Gastroenteritis, hemorrhagic diarrhea, Lipopolysaccharide-induced septic shock [23], deep wound infections, osteomyelitis, respiratory infections, bacteremia [22], enteric pathogenicity [21,24], nosocomial transmission
Acinetobacter baumannii *		Multi drug resistant—penicillins, cephalosporins, fluroquinolones, and aminoglycosides [25], multi drug resistant above plus carbapenem
Escherichia coli *		Carbapenem, third-generation cephalosporin [2]
Pseudomonas aeruginosa *	Broad intrinsic antimicrobial resistance, efflux pump, extended spectrum beta lactamase production, HGT, psychrotrophic [21]	Multidrug resistant, carbapenem, aminoglycosides, cephalosporins	Fatalities, nosocomial infections—urinary tract infections (UTIs), bacteremia, chronic airway infection in cystic fibrous patients [26]
Neisseria gonorrhea **	Cumulative chromosomal mutations in different genes related to cell wall biosynthesis [27], TetM protein conferring tetracycline resistance [28]	Azithromycin, third-generation cephalosporins, fluoroquinolones, sulfonamides, penicillin, tetracycline [27]	Gonorrhea, sexually transmitted disease (STI) and drug resistance
Salmonella species **	Gene mutation—DNA gyrase, efflux pump [29], alterations to outer membrane proteins [30], extended-spectrum cephalosporinases	Fluoroquinolone ciprofloxacin [29], ampicillin, chloramphenicol, sulfamethoxazole–trimethoprim tetracycline and streptomycin [31]	Foodborne disease, gastroenteritis, enteric fever, typhoid [32]
Helicobacter pylori **	Cytotoxin-associated gene A (cagA) [33] mutations, DNA gyrase [34]	Clarithromycin, metronidazole, levofloxacin [33], amoxicillin, and tetracycline [35]	Peptic ulcer disease, lymphoma, gastric adenocarcinoma [35]
Gram-positive
VRE **	β-lactamase, RNA methyltransferase, mutations in genes altering membrane structure [36]	Vancomycin, ampicillin, cephalosporins, aminoglycosides, daptomycin [36], low levels of intrinsic resistance to the quinolones	Nosocomial UTIs, immunosuppressed persons, bacteremia, bacteriuria [37], endocarditis, peritonitis [37]
MRSA **	Heat-stable staphylococcal enterotoxin production [21], altered penicillin-binding proteins (PBPs) [38]	Methicillin, amoxicillin, penicillin, oxacillin, cephalosporins, intrinsically resistant to the carbapenems [39]	Toxic shock syndrome, pneumonia, mastitis impetigo, cellulitis, osteomyelitis, endocarditis, bacteremia [38]
Clostridioides difficile	Erythromycin ribosomal methylase (erm) gene [40]	Aminoglycosides, lincomycin, tetracyclines, erythromycin [40], clindamycin, penicillin, cephalosporins, fluoroquinolones [41]	Nosocomial mortalities, pseudomembranous colitis, toxin-mediated disease [41]
Streptococcus pneumoniae ***	The erm(B) gene, altered PBPS [42], mutations of DNA gyrase gene, tet(M) and tet(O) genes [43]	Beta-lactam antibiotics [42], macrolides, lincosamides, fluoroquinolones, tetracyclines, trimethoprim–sulfamethoxazole [43]	Community-acquired pneumonia, meningitis, sepsis, bacteremia, and otitis media [43]
Acid Fast
Mycobacterium tuberculosis	Mutations in the embB gene [44], mutations in the pncA gene [45]	Ethambutol, rifampicin, isoniazid, pyrazinamide [45]	Tuberculosis (TB), multi drug resistant TB [44]
Mycobacterium avium complex (M. avium, M. intracellulare)	Lipid-rich cell wall [46], gene mutations in PBPs, embB, embR, rpsL [47], efflux pump, β-lactamases	Intrinsic multidrug resistance [46], macrolides [48], clarithromycin [48]	Mycobacterium avium-intracellular infection, lung disease [49], disseminated infection (usually associated with AIDS), lymphadenitis, localized cutaneous infection with tenosynovitis [47]

* Critically important pathogens on the WHO priority pathogen list. ** Highly important pathogens on the WHO priority pathogen list. *** Medium importance pathogens on the WHO priority pathogen list.