MORPHOLOGYPatientswithantibody,complement,orneutrophildefectsmayacquireseverelocalbacterialinfectionsthatdonotelicitanysignificantneutrophilicinfiltrate.Inthesepatients,theidentityofthecausativeorganismdeterminedbycultureorspecialstainsmaynothavebeenpredictedfromhistology.Althoughmanyviralcytopathiceffects(e.g.,cellfusionorinclusions)(see
Fig.
9.1 )maystillbepresent,viralinfectionsinimmunocompromisedhostsmaynotengendertheanticipatedmononuclearinflammatoryresponse.InpatientswithAIDSwhohavenohelperTcellsandcannotmountnormalcellularresponses,organismsthatwouldotherwisecausegranulomatousinflammation(e.g.,M. avium complex)failtodoso(
Fig.9.12
Fig. 9.12 IntheabsenceofappropriateT-cell–mediatedimmunity,granulomatoushostresponsedoesnotoccur.Mycobacterium avium infectioninaduodenalbiopsyfromapatientwithAIDS,showingmassiveintracellularmacrophageinfectionwithacid-fastorganisms(filamentousandpinkinthisacid-faststainpreparation).Theintracellularbacteriapersistandevenproliferatewithinmacrophages,becausethereareinadequateTcellstomountagranulomatousresponse.AIDS,acquiredimmunodeficiencysyndrome.
(Courtesy of Dr. Arlene Sharpe, Department of Microbiology and Immunology, Harvard Medical School, Boston, Massachusetts.)
Innormal(immunocompetent)individuals,thepatternsofhostresponsesarefairlystereotypicalfordifferentclassesofmicrobes;theseresponsepatternscanbeusedtoinferpossiblecausalorganisms.
Neutrophil-richacutesuppurativeinflammationistypicalofinfectionswithmanybacteria(“pyogenic”bacteria)andsomefungi.
Mononuclearcellinfiltratesarecommoninmanychronicinfectionsandsomeacuteviralinfections.
M.
tuberculosis andcertainfungi.
Cytopathicandproliferativelesionsarecausedbysomeviruses.
Necrosisresultsfromtissue-damagingtoxinsproducedbymicrobessuchasC. perfringens.
Chronicinflammationandscarringrepresentthefinalcommonpathwayofmanyinfections.
Arvanitis M, Anagnostou T, Fuchs BB, et al: Molecular and nonmolecular diagnostic methods for invasive fungal infections, Clin Microbiol Rev 27:490, 2014. [A very useful review of the many methods available for laboratory detection of fungal infections.]
Belkaid Y, Segre JA: Dialogue between skin microbiota and immunity, Science 346:954, 2014. [An excellent review of interactions between the microbiome of the skin and the immune system.]
Bennett E, Dolan R, Blaser MJ: Mandell, Douglas, and Bennett’s principles and practice of infectious diseases, ed 8, Philadelphia, 2014, Elsevier Saunders. [A comprehensive reference for infectious diseases, including very good sections on microbial pathogenesis.]
Bergsbaken T, Fink SL, Cookson BT: Pyroptosis: host cell death and inflammation, Nat Rev Microbiol 7:99, 2010. [A good introduction to the inflammasome and pyroptosis, including their role in the response to infections.]
Blair JM, Webber MA, Baylay AJ, et al: Molecular mechanisms of antibiotic resistance, Nat Rev Microbiol 13:42, 2015. [An accessible, well-written review at the introductory level.]