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Bacteria have evolved to survive in diverse environments. They survive exposure to harsh chemicals including antibiotics, and they also survive difficult growth conditions. They have learned to “detoxify” harmful substances such as antibiotics. Antibiotic resistance can either be intrinsic or acquired. This implies that resistance in pathogens can be as a result of obtaining resistance genes/traits from other resistant bacteria in the environment through genetic transfer mechanisms such as conjugation, transduction and transformation. Antibiotic resistance acquired by this means is known as acquired antibiotic resistance. This type of antibiotic resistance is transferrable from one organism to another through plasmids or transposons, and they are obtained horizontally. However, resistance that is not acquired from other already resistant bacteria is known as intrinsic antibiotic resistance. This type of resistance is obtained vertically and can be passed on from the parent cell to its progeny during reproduction or cell division. Intrinsic resistance can occur following mutation in the parent cell or as a result of some natural physical or structural makeup of the organism that allows them to be naturally resistant to the onslaught of antimicrobial agents.     


Some bacteria are said to possess innate/intrinsic resistance against antibacterial action put forward by antibiotics. Others are even naturally resistant to the antimicrobial agent that they synthesize. For example, Streptomyces are antibiotic synthesizing bacteria normally isolated from the soil; and they have immunity to the antibiotic that they synthesize. Microorganisms that are inherently resistant mount a great ingenuity in devising means or ways of neutralizing the killing or inhibiting action of antibiotics directed towards them. This innate form of antibiotic resistance in bacteria shows the different variations in the structure of the cell envelope of the organism, which allows them to mount resistance against drugs. The outer cell membrane of Gram-negative bacteria, lack of antibiotic target site on bacteria and the influx-efflux mechanisms of some pathogenic bacteria are some natural ways by which microbes build up resistance against antimicrobial onslaught. It is a vertical means by which bacteria acquire resistance. Intrinsic or innate form of antibiotic resistance can occur by any one of the following route (Figure 1):

  • Spontaneous mutation in the chromosomal DNA of bacteria.
  • Accumulation of several point mutations in bacteria.
  • An evolutionary process occurring only under selective pressure e.g. prior exposure of bacteria to antibiotics.
Figure 1. Schematic illustration of mechanisms of resistance in pathogenic bacteria. Pathogenic bacteria that harbour antibiotic resistance genes have exceptional ability to ward-off the antimicrobial activity of antibiotics; and this ability allows these microbes to continue to grow and thrive even in the face of potent antimicrobial agents capable of killing or inhibiting their growth. Antibiotic resistant bacteria have some notable features that allow them to make inefficacious the killing or inhibitory activity of antimicrobial agents directed at them. Some of these properties include: presence of efflux pumps (that pumps out antimicrobial agents from the cell); antibiotic degrading enzymes (that degrade antibiotics before they reach their target site on the pathogen); antibiotic resistance genes (that gives pathogenic bacteria the ability to express antibiotic resistance factors); antibiotic altering enzymes (that modify antibiotics before they reach their target site on the target organism); and presence of plasmids (that carry antibiotic resistance traits or factors). Photo courtesy: http://textbookofbacteriology.net/resantimicrobial_3.html


This type of antibiotic resistance is acquired by bacteria from the environment or other microorganisms by one of the means of genetic transfer such as conjugation, transformation, and transduction. In acquired/phenotypic resistance, the bacteria acquire reduced susceptibility to antibiotics through adaptation to growth within a specific environment. Pathogenic bacteria can develop resistance to a particular drug after prior unwarranted exposure to antimicrobial agents. In such scenarios, the microbes are taught to remain active even in the face of potent antimicrobial onslaught. Acquired resistance is a horizontal means by which bacteria become resistant to antibacterial properties of antibiotics. This form of antibiotic resistance can be achieved in bacteria by one of the following route (Figure 1):

  • Resistance can be maintained on horizontal mobile elements like plasmids, integrons and transposons which can be transferred to susceptible organisms.
  • Resistant genes can be transferred among bacteria through means of genetic transfer.
  • Resistance genes can be integrated into the bacterial chromosome or can be maintained in an extra chromosomal state (e.g. plasmids).
  • Changes in the genome of an organism (i.e. mutation) can predispose an organism to becoming resistant to antimicrobial agents.  


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