The most common bacterial species isolated from patients in the COVID-19 ICU are gram-negative isolates such as A. baumannii and K. pneumoniae, while high proportions of MRSA and VRE were observed among the gram-positive isolates [
66,
67,
68]. Numerous reports indicate that multidrug-resistant K. pneumoniae is one of the most important infectious agents isolated from COVID-19 ICU patients [
69,
70]. In these patients, co-infection with this pathogen causes a wide range of diseases, including pneumonia, urinary tract infections, bloodstream infections and sepsis [
70]. It is also the most common bacterial species isolated from non-CoVID-19 ICU patients [
7]. Co-infections with carbapenemase-resistant Klebsiella pneumoniae are very difficult to treat and have been associated with deterioration in the overall health status of COVID-19 ICU patients [
70]. Modification of drug binding sites, efflux pumps, biofilm formation and transposon acquisition of resistance genes are key components of the major mechanisms of antibiotic resistance in MDR K. pneumoniae [
71]. Several previous studies in many countries have shown that A. baumanni is the most common cause of respiratory infections in COVID-19 patients [
72,
73]. Co-infection with A. baumannii MDR significantly increases morbidity and mortality, especially in COVID-19 patients in intensive care [
73]. In addition, this bacterium has been implicated in several outbreaks in healthcare facilities [
24,
74]. Inappropriate use of personal protective equipment, poor adherence to hand hygiene protocols and irresponsible use of antibiotics are among the mainfactors contributing to the outbreak of this nosocomial MDR organism [
74]. As with K. pneumoniae, several studies worldwide have reported that A. baumanni strains with complete resistance to all tested antibiotics except colistin are increasingly being isolated from blood samples collected from ICU patients [
8,
72]. E. coli and P. aeruginosa were the other most common bacterial species in ICU patients, although different prevalence and antimicrobial resistance profiles of bacterial co-infections in COVID-19 patients have been reported worldwide [
75,
76].
Although rare at the time of admission, bacterial infections, particularly those caused by Pseudomonas aeruginosa, Klebsiella pneumoniae and Staphylococcus aureus, are common during prolonged hospital stays [
77]. Previous studies have shown that most patients admitted with COVID-19 do not require initial antibacterial therapy, which is only indicated in critically ill or severely immunosuppressed patients or those with diagnostic tests compatible with bacterial pneumonia [
27,
49,
53]. Blood and respiratory tract cultures may be considered in patients with severe disease who are admitted to intensive care with intubation for respiratory failure. In these patients, empirical antibacterial therapy with beta-lactam agents should be considered [
27,
53]. A reassessment to discontinue this therapy if microbiological results are negative or to initiate targeted antibacterial therapy according to the identified pathogen should be performed after 48-72 hours, according to the American Thoracic Society (ATS) guidelines [
78]. S. aureus, S. pneumoniae and H. influenzae are the most common respiratory and bloodstream bacterial pathogens causing co-infection in COVID-19 patients [
27]. Other bacteria involved in this infection, although less common, are MRSA and P. aeruginosa [
79]. A combination of beta-lactams and macrolides is used to treat these co-infections [
79]. Although guidelines recommend the use of combination therapy over monotherapy to avoid the risk of selecting resistant strains, the former gives better results than the latter when given to patients infected with atypical pathogens such as L. pneumophila [
27]. Anti-MRSA therapy should be discontinued if this microorganism is not isolated from nasal swab cultures of critically ill ICU patients [
79]. Pneumonia and bloodstream infections (BSIs) were the most common causes of nosocomial bacterial infections in COVID-19 ICU patients. Major risk factors for nosocomial bacterial infections in COVID-19 patients were hypoxia on admission, need for mechanical ventilation and ICU admission within 2 days [
5]. Coagulase negative staphylococci, Enterococcus spp, Klebsiella pneumoniae, P. aeruginosa and S. aureus were identified as the main pathogens responsible for nosocomial bacterial infections in COVID-19 patients [
24]. COVID-19 patients with nosocomial bacterial infections had worse outcomes than those without nosocomial bacterial infections [
80]. The former had almost twice the mortality rate of those without nosocomial bacterial infections [
81]. Numerous studies worldwide have reported that COVID 19 patients requiring prolonged hospitalisation and mechanical ventilation develop multi-resistant nosocomial infections caused by carbapenemase-resistant Gram-negative bacteria, including K. pneumoniae, A. baumanii, Enterobacter cloacae and E. coli [
67,
82]. The rapid and widespread increase in carbapenem-resistant Gram-negative infections in COVID 19 patients has been attributed to noncompliance with standard and contact precautions, overworked and inexperienced ICU staff, and lack of screening for CRE to prevent patient-to-patient transmission [
83]. These antimicrobial-resistant infections are difficult to treat and lead to longer hospital stays and higher healthcare costs. The excessive and non-compliant use of antimicrobial agents (e.g. patients taking the wrong or unnecessary antibiotics) causes drug-sensitive bacteria to be killed, while drug-resistant strains persist. These then multiply and accelerate the growth of antimicrobial resistance [
84]. As the search for new classes of antibiotics is bleak and only a few antibiotics may reach the market in a few years, alternative strategies must be developed and diagnostics improved to prevent the further spread of drug resistance. Several promising alternatives have been proposed to combat the growing threat of antibiotic-resistant bacteria [
85]. These include inorganic nanoparticles, bacteriophages, antimicrobial enzymes, peptides and small molecules [
86]. Inorganic nanoparticles (NPs) act by causing pores in the cell membrane, leakage of the cytoplasm and disruption of the electron transport chain. Nanoparticles of silver or zinc oxide have been used successfully to combat pathogenic bacteria and fungi [
86]. Although inorganic NPs have promising potential for many industrial and commercial applications, including medicine, further studies are needed to determine their effects on ecosystems and human health. Although the use of bacteriophages as antibacterial agents dates back to the early 1900s, phage therapy was eclipsed by the discovery of antibiotics [
87]. However, in recent years there has been a resurgence of interest in bacteriophage therapy as a result of the increase in antibiotic resistance [
88]. In this context, the use of bacteriophages as new experimental drugs for the treatment of patients who do not respond to available antibiotics has been approved by the FDA in a number of cases [
89,
90]. A phage cocktail, consisting of several phages that infect different types of pathogens, has been successfully used to treat life-threatening infections in humans [
91]. It is therefore possible that phage therapy could be of great benefit in the fight against secondary infections in critically ill patients with COVID-19. However, to support the routine clinical use of phage therapy, further clinical research studies are needed [
92]. Phage lysins, also known as “enzybiotics', are bacteriophage-encoded lytic enzymes capable of bacterial cell wall degradation [
93]. The lytic activity of lysins against MRSA, VRSA and S. pneumoniae infections has been demonstrated in several studies [
94]. Of particular interest is the synergy between antibiotics and phage lysins in the treatment of pneumococcal bacteraemia caused by multi-resistant Streptococcus pneumoniae [
95]. The antimicrobial activity of various small molecules targeting different cellular structures, such as cell wall lipid intermediates, the cytoplasmic membrane and bacterial RNA polymerase, has been reported in numerous articles [
96,
97]. The ease with which they can be synthesised and modified makes these small molecules promising candidates for the treatment of antibiotic-resistant infections.