Introduction to Microbes and Host-pathogen Interactions

Transcription

1 Intrductin t Micrbes and Hst-pathgen Interactins A micrbe is a micrscpic rganism, which may be single-celled r multicellular. These include prkarytes rganisms that lack a cell nucleus and ther membrane bund rganelles, which are almst always unicellular and eukarytes rganisms which has a cell nucleus and ther membrane bund rganelles. There are three majr differences between prkarytic and eukarytic cells: Structure f nucleus: prkarytic nuclear material is nt surrunded by a membrane whilst eukarytic nuclear materials are Intracellular rganelles: eukarytes cnsist f membrane bund cellular rganelles whilst prkarytes d nt Cell wall structure: there are many unique cmpunds in bacterial cell walls, such as peptidglycan, lipplysaccharides and D-amin acids These structural and bichemical differences between prkarytic and eukarytic cells are f critical imprtance in design f antimicrbial agents f antibitics as it ensures selective txicity t these antibitics. Fr example, penicillins inhibit the synthesis f peptidglycan, an essential structural cmpnent f bacterial cell wall. As it is nt fund in human cells, it is a gd target fr selective txicity. There are three dmains f life: eukarya, bacteria and archaea. Eukarya includes all eukarytic rganisms, and thus cntains fur kingdms f rganisms Animalia: multicellular animals Plantae: multicellular plants Fungi: multicellular fungi and unicellular yeast Prtsita: unicellular algae and prtza Bacteria is a dmain which is prkarytic Archaea is a diverse prkarytic grup r rganism that ften live under extreme envirnmental cnditins, such as high salt/temperature, ht springs in New Zealand

2 Archaea It is a dmain f single-celled micrrganisms that are simple in frm and nly grw in xygen-free envirnments. They are thermphiles and live in extreme weather cnditins. Archaea and bacteria are generally similar in size and shape. They als have similar hereditary mechanisms, have a cell membrane cmpsed f lipids, a cell wall and has a metablism based n ATP. Hwever, they d nt cnsist f a peptidglycan in their cell wall, have unusual lipids in cell membrane and has gene and ribsme structure similar t that f eukarytes They include rganisms that prduce methane frm CO 2 and H 2, such as methangens which btain energy frm this prcess and are als a surce f majr prprtin f ur natural gas reserves. Additinally, they als include rganisms that can (1) survive and grw in ht, acidic envirnments (thermacidphiles), such as ht springs and deep hydrthermal vents, ften using H 2 S as an energy surce and (2) tlerate high salt cncentratins (halphiles), such as the Dead Sea. Similarities with Bacteria and Eukarya Cmmn features with bacteria Anucleate n defined, membrane-bund nucleus Cell envelpe Generally pssess single circular chrmsme Vary shape and size Cmmn features with eukarya DNA replicatin, transcriptin and translatin is mre similar t eukarya than t bacteria Archea DNA has histnes (mdern drug targets) DNA binding prteins cnsidered t be a defining characteristic f eukarya Many enzymes invlved in DNA replicatin (DNA plymerase, primase etc. are mre similar t thse fund in the eukarya than t bacteria) Differences with Bacteria and Eukarya Distinctive rrna sequences which are resistant t heat, xidating agents and are very stable Plasma membrane different t bacteria and eukarya and cntain unique membrane lipids Archaea lack peptidglycan: key cmpnent f the cell wall f bacteria Outer layer f the cell wall S layer (structured r surface layer) interacts directly with the plasma membrane, frming a cmplex lattice Unlike bacteria, there are virtually n knwn bnafide pathgens f humans. The diversity f the Archaea in the human bdy is als substantially lwer, including representatives f nly ne phylum (Euryarchaeta) Methanbrevibacter smithii gut and vagina Methansphaera stadtmanae - gut Methansphaera ralis muth

3 Methansphaera ralis M. ralis is a majr archaeal player in the ral cavity. Its prprtins increase with the severity f ral diseases/infectins, such as peridntal diseases. Apical peridntitis is the result f the infectin f a tth s rt canal which can be invaded by ral micrflra, such as M. ralis. Methansphaera stadtmanae and Methanbravibacter smithii M. smitihii is the majr archaeal cmpnent in the human gut system whilst M. stadtmanae is a less frequently detected species. They bth are methangens, micrrganisms that prduce methane as a metablic byprduct in anxic cnditins, and thus, disrders invlved include inflammatry bwel disease, irritable bwel syndrme and clrectal cancer. In cntrast, they can als aid in decreasing cardivascular disease. Trimethylamine (TMA) is xidised t TMAO t prmte athersclersis, a disease in which plaque builds up inside the arteries, narrwing and hardening it. Archaea uses TMA as a carbn surce, thus reducing the prductin f TMAO and, cnsequently, cardivascular risks. They als aid in preventing Trimethylaminuria (TMAU), fish dur syndrme, a rare metablic disrder that causes a defect in the nrmal prductin f Flavin-cntaining mnxygenase 3 (FMO3), resulting in fishy sweat, breath and urine. Flavin mnxygenase is respnsible fr cleaving ut TMA in the liver, hwever, when a defect ccurs in this enzyme, TMA is unable t be cleaved ut f the liver, resulting in TMAU. Similar, archaea uses TMA, thus reducing the amunt f TMA in the liver and preventing TMAU.

4 Bacteria Bacteria is a large grup belnging t prkarytic micrrganisms. They are free-living and can vary in size, frm 0.1µm t 50µm in diameter. Due t the small size, and the lack f cntrast between cells and backgrund, it makes it difficult t bserve. Therefre, it is helpful t (1) use high pwered 100x il immersin lens cupled with 10x eyepiece which gives 1000x magnificatin and (2) stain cells with clred dye fr a greater cntrast between the cell and the backgrund. There are imprtant differences between bacteria and eukarytes and are ften cmpared t what the prkarytes lack in: Membrane bund nucleus Internal membranus structures (ER, glgi) A cytskeletn (actin, spectrin, prtein) Hwever, sme bacteria d have their DNA enclsed in a membrane (Planctmycetes), sme d have membrane-bund rganelles and extensive intracytplasmic membrane structures and sme d have cytskeletal elements. Nevertheless, they will cntinue t be differences between prkarytes and eukarytes as a general statement. Bacterial mrphlgy Ccci (spherical) Staphylcccus aureus is a cmmn bacterium that lives n the skin r in the nse, thus is cnsidered nrmal flra. An infectin f this, hwever, is difficult t treat in humans. Bacilli (rd-shaped) Clstridium difficile Spirchaete (spiral) Trepnema pallidum is the cause f fd-brne infectins, such as fd pisning Leptspira spp. Camplybacter jejuni Cmmas s (curved) Vibri chlera Sme bacteria are a mixture f the tw, such as: Ccc-bacilli is a type f bacterium with a shape intermediate between ccci and bacilli. It is shrt, rund and rd-shaped. Yersinia pestis is a rd shaped cccbacillus, and a facultative anaerbic rganism that can infect humans, especially indigenus Australians. Acinetbacter baumannii is a shrt, rund, rd shaped bacterium that is prly understd. It can be an pprtunistic pathgen in humans, affecting peple with cmprmised immune systems.

5 Plemrphic bacteria are able t alter their shape r size in respnse t envirnmental cnditins. Mycplasma and Ureaplasma They are bacteria cmmnly fund in the reprductive tract f bth men and wmen They are the smallest species f bacterial cells N rigid cell walls arund the cell membrane, therefre, they are difficult t get ff Rickettsia A diverse grup f bacteria, sme f which can be transmitted t humans via the bites f fleas, lice, ticks r mice Present as ccci, rds r thread-like Phylgenetically between bacteria and viruses and are bligate intracellular parasites, therefre, they must live inside the cell Cause imprtant vectr brne (ticks/fleas) diseases, such as typhus, scrub typhus and rcky muntain sptted fever Structural Features Bacteria have many specialised structures that are imprtant fr survival, interactin with hst, virulence and pathgenesis. Flagella Thin, invisible under light micrscpy unless special stains used Peritrichus r plar (rientatin) Mtility rtates like a prpeller Nt all bacteria have flagella, and therefre, nt all are mtile The arrangement f flagella can have ne, r multiple, depending n the number f flagella and arrangement are terms as: Mntrichus bacteria with a single flagellum Amphitrichus bacteria with a flagellum at bth ends Peritrichus flagella that cver the entire surface f the bacterium Plar flagella nly at ne end r anther Pili r fimbriae Shrter than flagella and mre numerus, expsed n the cell surface Cmpsed f prtein subunits (pilin) Nt invlved in mtility, but rather imprtant in pathgenesis mechanism which causes diseases

6 Main functin is in adhesin allws bacteria t adhere t mammalian cells Pili allw adherence t epithelial cells f intestine e.g. Escherichia cli traveller s diarrhea Capsules/S-layer (glyccalyx) Many bacteria secrete a slimy layer nt their cell surface Capsule, slime layer, glyccalyx Usually plysaccharide cmplexes Different cmpsitin in different bacteria Als imprtant in pathgenesis Adhesin f bacteria e.g. Streptcccus mutans dental plaque Preventin f phagcytsis by hst cells (a defence mechanism against immune respnse which makes it difficult fr macrphage t underg phagcytsis) e.g. Streptcccus pneumniae pneumnia Prtectin against desiccatin, prevents bacteriphages frm attachment and infectin, prtects against phagcytsis Frms bifilms cmplexes f micrbial cmmunities embedded in a capsular material mass Bifilm frmatin is an imprtant virulence factr clnisatin f indwelling devices assists ral bacteria t clnise teeth resulting in decay Outer membrane (-ve bacteria) Cmplex utside later f the cell wall, imprtant in pathgenesis Cell wall Outside the plasma membrane that enclses the bacterial cytplasm Rigid, determines shape Peptidglycan Peptidglycan is a plymer f amin acids and sugars that makes up the cell wall f all bacteria. As humans d nt have the enzymes t break dwn peptidglycan, this gives bacteria a survival advantage. The glycan is cmpsed f tw alternating sugars: N-acetyl glucsamine (NAG) and N- acetylmuramic acid (NAM), bth being derivatives f glucse. NAG and NAM are cvalently linked in lng chains, alternating with ne anther. On the ther hand, peptid is cmpsed f fur amin acids tetrapeptides cntaining bth D- and L-amin acids, als in an alternating pattern. It cnsists f three amin acids that are nt fund in ther prteins and are nly fund in bacteria peptidglycan: D-glutamic acid, Diaminpimelic acid and D-alanine,

7 respectively. The terminal D-alanine crss links with anther NAM either directly (+ve gram) r indirectly (-ve gram). In regards t linking indirectly, the peptide interbridge spaces the peptidglycan by 5 peptide residues. There are als variatins t crss-linking plymers. Mycbacteria such as M. tuberculsis links L-Lys and L-Lys directly with n bridge, even with the lack f terminal D-alanine There can be different amin acids r different numbers f amin acids in the bridge, such as fr S. pneumniae links D-Ala D-Ala directly D-Ala-Ser-D-Ala Tertiary structure f peptidglycan cnsists f NAG and NAM linked with a tetrapeptide interactin. It can be the site f actin f penicillin and ther beta-lactam antibitics. Bacterial Cell Walls The differences in peptidglycan frm the basis f the Gram strain, differentiating them int tw large grups (Gram psitive and Gram negative), depending n their ability t retain crystal vilet/idine cmplex after alchl wash.