How One of Switzerland’s Oldest Banks Became a Meme Stock

“Credit Suisse is probably going bankrupt.”

It was Saturday, Oct. 1, and Jim Lewis, who frequently posts on Twitter under the moniker Wall Street Silver, made that assertion to his more than 300,000 followers. “Markets are saying it’s insolvent and probably bust. 2008 moment soon?”

Mr. Lewis was among hundreds of people — many of them amateur investors — who had been speculating about the fate of Credit Suisse, the Swiss bank. It was in the middle of a restructuring and had become an easy target after decades of scandals, failed attempts at reform and management upheavals.

There seemed to be no immediate provocation for Mr. Lewis’s weekend tweet other than a memo that Ulrich Körner, the chief executive of Credit Suisse, had sent employees the day before, reassuring them that the bank was in good financial health.

But the tweet, which has been liked more than 11,000 times and retweeted more than 3,000 times, was one of many that helped ignite a firestorm on social media forums like Twitter and Reddit. The rumor that Credit Suisse was in trouble ricocheted around the world, stumping bank executives and forcing them to call shareholders, trading partners and analysts to reassure them that everything was fine before markets reopened on Monday.

Reached via private message on Twitter, Mr. Lewis said all he had looked at before sending out his tweet was Credit Suisse’s “low stock price and memes on Reddit.”

A storied institution had become a meme stock.

The same day Mr. Lewis tweeted that the bank might go bankrupt, Hunter Kikut tweeted: “High likelihood of Credit Suisse going bust. Will be shorting on Monday.”