Ciência Diária » Bactérias se tornam altamente altruístas na presença de antibiótico

Bactérias se tornam altamente altruístas na presença de antibiótico

Submitted by Ciência Diária on Thursday, 2 September 201053 Comentários

Crédito: Peter Arkle - HHMI.

O corpo luta contra os invasores e os invasores lutam contra o corpo. Quando o antibiótico chega para dar uma forcinha, bactérias mostram que sabem como ninguém trabalhar em grupo. De acordo com uma pesquisa realizada pelo Howard Hughes Medical Institute, alguns patógenos resistentes podem neste momento deixar o egoísmo de lado e proteger uma colônia inteira.

Acreditava-se que a resistência aos antibióticos funcionava em nível individual: uma bactéria sofria alguma mutação genética que conferia proteção contra o medicamento, abrindo o caminho para a sobrevivência e reprodução. Resultado: mutantes repovoavam o corpo com descendentes fortes.

Entretanto, um novo estudo publicado hoje pela revista Nature mostra que o ataque do antibiótico desperta uma reação em cadeia. A Escherichia coli resistente, por exemplo, produz uma proteína que desperta vários mecanismos de proteção em suas colegas “mais fraquinhas”. A descoberta pode ajudar a explicar por que bactérias resistentes podem dar um verdadeiro baile em médicos e profissionais de saúde pública.

Para chegar a essa conclusão, os pesquisadores usaram um biorreator – um grande vaso de vidro tampado com muitos braços que permitiam controlar o experimento – para cultivar bactérias de E. coli. Eles escorriam então o antibiótico norfloxacina, retirando amostras das bactérias para medir a concentração inibitória mínima – CIM – (uma forma de prever a resistência à droga, representando a concentração mínima de droga necessária para impedir o crescimento da colonia). Quanto mais alta a CIM, maior a resistência ao antibiótico.

Para surpresa geral, a equipe descobriu que o CIM de amostras individuais era menor do que as da população em geral. Os pesquisadores também observaram que bactérias com alto CIM eram poucas e, em geral, estavam distantes umas das outras – constituindo menos de 1% da população.

Então, o grupo analisou as proteínas produzidas por bactérias com alto CIM na presença do antibiótico, descobrindo que a tryptophanase era particularmente abundante. Ela quebra o aminoácido triptofano em componentes menores, liberando uma molécula sinalizadora que oferece dois tipos de proteção contra a droga: uma liga as “máquinas” que bombeiam o antibiótico para fora da célula; outro que protege contra o estresse oxidativo.

Bactérias “resistentes” são capazes de despertar o mecanismo de proteção geral, mesmo que tenha um alto custo individual. Este comportamento altruísta apresenta um paradoxo conhecido por biólogos evolucionistas: se a evolução favorece o mais forte, por que um indivíduo se sacrifica pelo resto do grupo? A explicação estaria na teoria de seleção de parentesco, segundo a qual organismos se comportam de forma altruísta para passar os genes para próximas gerações.

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