The findings of gram-negative infections are non-specific, not distinguishing them from other infectious diseases on physical examination. A classic example is pneumonia; its symptoms include fever, chest pain, dyspnea, and purulent expectoration; these symptoms may correlate with tachycardia, tachypnea, hypoxemia, and auscultatory signs of consolidation. However, these signs are not specific to the disease. Cultures are the only way to determine the organism.[25] Similarly, in bacterial meningitis, one may note fever, severe headache, nausea, vomiting, neck stiffness, prostration, and mental confusion. However, such features are not exclusive, and it requires other evidence to conclude the type of organism.[26]

Whereas the infections produced by gram-negative are practically indistinguishable from other etiologies, the laboratory characterization can define the organism and its antimicrobial susceptibility profile. Thus, the gold-standard examination of these infections are the cultures; however, they have the problem of a delay in the result. Another essential exam for bacterial isolates is the Gram stain, although simple, it can quickly distinguish the course of a drug intervention.  Enterobacteria, because of their heterogeneity, depend on various biochemical tests (Indol, Voges-Proskauer, catalase, cytochrome oxidase, urease, motility, citrate, ONPG, Dnase, decarboxylation of lysine, gas production among others), grown in primary isolation agar. Species such as E. coli, because they are fermenters of strong acids, they produce large quantities of mixed acids which make the pH of the media to present pink colonies. Others such as non-fermenters grow in primary isolation media (blood agar and MacConkey), in the OF-oxidation/fermentation medium - grow only in the aerobic environment.[9]

However, it is important to mention the identification of bacterial strains resistant to multiple drugs may require molecular methods, but they are not available in all laboratories. Phenotypic methods such as Modified Hodge (MHT) and Combined Diffusion Disc (CDT) using EDTA are a good alternative.[27] The MHT is a test based on the inactivation of carbapenems by bacterial strains containing the enzymes carbapenemases, enabling a susceptible strain to extend its growth to a disc containing the antimicrobial along the inoculum of the strain tested. This test is recommended for strains with high minimum inhibitory concentration (MIC) or reduced zones of inhibition in the disk.[28] The CDT comprises a test disc-diffusion with carbapenem antibiotics, where carbapenem discs are placed with and without EDTA, and their connection predicts whether the organism produces or does not carbapenemase.[29]

Treatment options for gram-negative MDR infections are limited, and the results are generally disappointing because of drug resistance. MDRs are still evolving, which provides resistance to novel antimicrobials. Thus, as the availability of new drugs progresses slowly, some previously abandoned options have reappeared, such as polymyxins and colistin that have high toxicity (nephrotoxicity and neurotoxicity). Furthermore, resistant genes to these drugs have been reported such as mcr-1 - cause additional concern. But it is known that the combination of these drugs with carbapenems may have an improved synergistic action.[30] Another drug of choice is tigecycline since it shows in vitro activity against MDR; however, there are limitations in its use as high doses are required, and tissue penetration is often poor, which impairs its action in vivo.[31]

Fosfomycin, an old drug used to treat urinary tract infections, has also resurfaced as a potential agent for the treatment of these MDR infections. However, monotherapy with this drug also results in resistance and its use is appropriate in conjunction with other agents such as carbapenems and polymyxin. This drug has moderate absorption, which restricts its use only for urinary tract infections.[32] Drugs once rejected because of their nephrotoxicity and ototoxicity, the aminoglycosides, have resurfaced again because of their effectiveness against gram-negative organisms. However, these drugs do not perform the same against MDRs except for urinary tract infections; they also exhibit less toxicity when compared to polymyxin and tigecycline and may be useful in combination therapy.[33]

The carbapenems are the last-choice drugs for ESBL-producing organisms; however, the appearance of carbapenemases has made treatment difficult. Monotherapy is rarely effective. However,  combination therapy with these drugs is effective and because of the high affinity of carbapenemases for ertapenem, making it a "bait" for the action of other carbapenems.

New drugs such as ceftazidime-avibactam and meropenem-varbobactam have emerged; the first is an antipseudomonal cephalosporin associated with a beta-lactamase inhibitor, while the second is a carbapenem with b-serine and has anti-lactamase activity. Early results are encouraging.[21]

• Gram-positive bacterial infections (Staphylococcus spp., Streptococcus spp., Micrococcus spp.)
• Viral infections (HIV, respiratory syncytial virus, influenzas)
• Mycobacterium spp. and Nocardia spp.
• Fungal infections (Cryptococcus spp, Aspergillus spp., Candida albicans)

Polymyxins have been in disuse since the 1970s because of their known toxicity, mainly because of their nephrotoxic effects. With the appearance of gram-negative MDR infections, they have revitalized. However, there are reports of problems with toxicity and increased resistance to these drugs.[34] The mechanism of renal toxicity exerted by these drugs occurs mainly through oxidative stress, resulting in mitochondrial dysfunction and loss of the membrane potential of this organelle. Apoptosis is also another effect attributed to this drug, which activates caspases 3, 8 and 9 of pulmonary epithelial cells.[35] The toxicity of polymyxins is related to concentration; however, the current pharmacokinetics and pharmacodynamics have already shown the dosage for this class of drugs is not the most appropriate, thus requiring new studies for correction, which could attenuate the toxicity of the drug.

Another class of drugs with toxic potential are the aminoglycosides which mainly cause ototoxicity and nephrotoxicity. These drugs block cationic channels of the ciliated cells in the inner ear. They have the same action with the mechanosensitive transduction channels of these cells, whereas inside these cells they promote biochemical changes that culminate in the elevation of intracellular calcium and the formation of reactive species of oxygen (EROS), which irreversibly damage the cell.[36] The nephrotoxicity of the aminoglycosides is mainly related to damage to the proximal tubule; the effects are relative to the dose and the time of exposure. The endocytosis of these drugs with consequent release of the lysosomes content is the mechanism by which the drugs exert their toxicity; also, they also block the calcium channels, culminating in the loss of this ion, magnesium, and potassium.[37]

Many variables are present in gram-negative infections; therefore, the prognosis of the diseases caused by these pathogens is difficult to measure and is not unanimous in the literature. However, such infections are usually associated with a poor outcome, mainly in elderly patients, with a history of comorbidities, those with solid organ transplantation and malignant diseases. Treatment success also depends on the causative microorganism and the site of infection. Serious infections with high mortality rates are related to delayed antibiotic therapy or inadequate therapy while appropriate antibiotic administration correlates with patient survival. Besides that, the lack of a well-defined protocol for these infections may lead to treatment failure, since suboptimal or inadequate administration contributes to poor results. Finally, monotherapy is also ineffective, especially when compared to combination therapy, which is related to good prognosis.[38]

Several complications may result from gram-negative infections, especially the enzyme producers that hydrolyze carbapenems. A concerning infection due to recurrence and ease of acquisition is the urinary tract, as it is increasingly common to find multidrug-resistant organisms in the community, and such infections, if not treated properly, could lead to renal failure, sepsis, and even death.[20] Similarly, the nosocomial infections in frail patients with suppressed immunity, comorbidities, and transient immunosuppressive status, make this combination a challenge for health professionals.[21]

In this sense, special care is necessary with patients who have suffered burns, because as there are more ports of entry for microorganisms; infections in burn patients can be potentially lethal and invasive depending on the microbial load and the pathogen itself.[39] Another risk is respiratory tract infections, mainly associated with mechanical ventilation, since gram-negatives are responsible for such infections, with Enterobacteriaceae and non-fermenters involved, with considerable potential for fatality.[40]

One of the chief difficulties in the treatment of multiresistant gram-negative infections is the excessive use of antibiotics, not only those acquired by the community but also in hospitals. As the MDRs became epidemic, it is observable that overuse of these antimicrobials was one of the causes that urgently requires addressing. Thus, measures aimed at changing the use of these drugs, with educational campaigns, as well as the fight against self-medication with practices such as monitoring of drug consumption and their registration in pharmacies are necessary to change the habit of the population and also health professionals.

The problem of multiresistant GNB infections is on the rise since the beginning of the 21st century. However, there is difficulty eradicating drug-resistant organisms because of the lack of effective antibiotics. Thus, simple measures such as regular hand hygiene, adequate sterilization of medical equipment, isolation of patients suspected or diagnosed with MDR microorganisms - especially those submitted to invasive procedures - with caution regarding the entry of external persons. All individuals should abide by infectious disease preventive rules established in the healthcare setting. Another significant action is the laboratory worker who isolates MDR pathogens to immediately inform the epidemiological surveillance the staff of the health establishment so that prevention and control measures are enacted promptly.

Results

In summary, the results at present are still not good, especially in elderly patients with a history of multiresistant BGN infection; however, the aforementioned measures, although simple, could save lives as well as avoid the spread of these MDR bacterial strains which pose a high risk to society for the reduced spectrum of drugs that address these infections, which, in addition to being often inefficient, also present high levels of toxicity.